Published: November 7, 2018 by Ken Feldman
Assignable cause, also known as a special cause, is one of the two types of variation a control chart is designed to identify. Let’s define what an assignable cause variation is and contrast it with common cause variation. We will explore how to know if your control is signaling an assignable cause and how to react if it is.
Overview: What is an assignable cause?
A control chart identifies two different types of variation: common cause variation (random variation resulting from your process components or 6Ms ) and assignable or special cause variation.
Assignable cause variation is present when your control chart shows plotted points outside the control limits or a non-random pattern of variation. Since special cause variation is unexpected and due to some factor other than randomness, you should be able to assign a reason or cause to it.
When your control chart signals assignable cause variation, your process variable is said to be out of control, or unstable. Assignable cause variation signals can be identified by use of the Western Electric rules, which include:
- One point outside of the upper control limit or lower control limit
- A trend of 6 or 7 consecutive points increasing or decreasing
- A cycle or repeating pattern
- A run of 8 or more consecutive points on either side of the average or center line.
Assignable cause variation can be attributed to a defect, fault, mistake, delay, breakdown, accident, and/or shortage in the process. Or it can be a result of some unique combination of factors coming together to actually improve the process. When assignable causes are present, your process is unpredictable. The proper action and response is to search for and identify the specific assignable cause. If your process was improved as a result of your assignable cause, then incorporate it so that the cause is retained and improvement maintained. If your process was harmed by the assignable cause, then seek to eliminate it.
3 benefits of an assignable cause
Assignable causes can be good or bad. They are signals that something unexpected happened. Listen to the signal.
1. Signals something has happened
Special or assignable cause variation signals that something unexpected and non-random has occurred in your process.
2. Specific cause
By investigating and identifying the specific cause of your signal, you can narrow in on your next steps for bringing the process back into control.
3. Can become common cause variation
Good news! You found that your assignable cause for lowered production was due to a power outage. Unfortunately, you may not be able to stop power outages in your community. If nothing is done, your assignable cause becomes a common cause.
You might not be able to stop power outages, but could you install a back-up generator? Then, if the generator doesn’t kick on, you will have an assignable cause you can do something about.
Why is an assignable cause important to understand?
Interpreting what an assignable cause tells you is important to understand.
Provides direction for action
Since an assignable cause can be a signal of something good or bad, you need to understand the different actions. Don’t ignore special or assignable causes.
Not every unusual point has an assignable cause
While at your favorite casino, you may throw a pair of dice at the craps table. Is there an assignable cause for throwing an 11 or a 10, or is it random variation? No, you would expect the process of rolling a fair pair of dice to show 10s and 11s. What about a 13? That would be unexpected and probably the result of something unusual happening with the dice. The same is true for your process. Don’t assume an assignable special cause unless your control chart signals it.
Useful for determining whether your improvements worked
When you improve the process, your control chart should send signals of special cause variation — hopefully in the right direction. If you can link that signal to the specific assignable cause of your improvement, then you know it worked.
An industry example of an assignable cause
The accounts receivable department of a retail chain started to get complaints from its customers about overbilling. Fortunately, the manager of the department had participated in the company’s Lean Six Sigma training and had been using a control chart for errors.
Upon closer review, she noticed that errors seemed to occur more on Fridays than the rest of the week. In fact, the chart showed that almost every Friday, the data points were outside the upper control limit. She was concerned that nobody was identifying that as a signal of special cause.
She put together a small team of clerks to identify why this was happening and whether there was an assignable reason or cause for it. The assignable cause was determined to be the extra work load on Fridays.
The team recommended a change in procedure to better balance the workload during the week. Continued monitoring showed the problem was resolved. She also held an all-hands meeting to discuss the importance of not ignoring signals of special cause variation and the need to seek out an assignable cause and take the appropriate action.
3 best practices when thinking about an assignable cause
Signals of special cause variation require you to search for and identify the assignable cause.
1. Document your search
If you’ve identified the assignable cause, document everything. If this cause happens again in the future, people will have some background to act quickly and eliminate/incorporate any actions.
2. Quickly identify the cause
Time is of the essence. If the cause is resulting in a deteriorating process, act quickly to identify and eliminate the cause. The recommendation is the same if your cause made the process better, otherwise, whatever happened to improve the process will be lost as time goes by.
3. Don’t ignore signals of assignable cause
Even if you get a single signal of special cause, search for the assignable cause. You may choose not to take any action in the event it is a fleeting cause, but at least try to identify the assignable cause.
Frequently Asked Questions (FAQ) about an assignable cause
1. is an assignable cause always bad .
No. It is an indication that something unexpected happened in your process. It could be a good or bad thing. In either case, search for and identify the assignable cause and take the appropriate action.
2. What are some sources of an assignable cause?
Some sources may be your process components such as people, methods, environment, equipment, materials, or information. Your process variation can come from these items and can be the assignable cause of a signal of special cause variation.
3. How do I tell if I should look for an assignable cause?
Control charts were developed to distinguish between common and special cause variation. If they signal special cause variation in your process, seek out an assignable cause and take the appropriate action of either eliminating or incorporating your assignable cause.
Final thoughts on an assignable cause
All processes will exhibit two types of variation. Common cause variation is random, expected, and a result of variation in the process components. Special cause variation is non-random, unexpected, and a result of a specific assignable cause.
If you get a signal of special cause variation, you need to search for and identify the assignable cause. Once found, you will either seek to incorporate or eliminate the cause depending on whether the cause improved or hurt your process.
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Statistical Process Control (SPC)
Posted by Ted Hessing
Statistical Process Control (SPC) is a statistical method to measure, monitor, and control a process. It is a scientific visual method to monitor, control, and improve the process by eliminating special cause variations in a process.
History of Statistical Process Control (SPC)
SPC has been used in Western industries since 1980, but it started in America’s twenties. Walter A. Shewhart developed the control chart and the concept that a process could be in statistical control in 1924 at Bell Telephone Laboratories, USA. Likewise, the SPC concepts were included in the management philosophy by Dr. W.E. Deming just before World War II. However, SPC became famous after Japanese industries implemented the concepts to compete with Western industries.
Meaning of SPC
- Statistics : Statistics is a science that deals with the collection, summarization, analysis, and drawing of information from the data.
- Process: It converts input resources into the desired output (goods or services) with a combination of people, materials, methods, and machines, as well as measurements.
- Control: System, policies, and procedures in place so the overall output meets the requirement.
Why use Statistical Process Control
Today companies face increasing competition and operational costs, including raw materials increasing. So, it is beneficial for organizations to have control over their operation.
Organizations must try to continuously improve quality, efficiency, and cost reduction. Many organizations still follow inspection after production for quality-related issues.
SPC helps companies move towards prevention-based quality control instead of detection-based quality controls. By monitoring SPC graphs, organizations can easily predict the behavior of the process.
Statistical Process Control Benefits
- Reduce scrap and rework
- Increase productivity
- Improve overall quality
- Match process capability to product requirement
- Continuously monitor processes to maintain control
- Provide data to support decision-making
- Streamline the process
- Increase in product reliability
- Opportunity for company-wide improvements
Statistical Process Control Objective
SPC focuses on optimizing continuous improvement by using statistical tools to analyze data, make inferences about process behavior, and then make appropriate decisions.
The basic assumption of SPC is that all processes are generally subject to variation. To that end, Variation measures how data are spread around the central tendency. Moreover, variation may be classified as one of two types, random or chance cause variation and assignable cause variation.
Common Cause: A cause of variation in the process is due to chance but not assignable to any factor. It is the variation that is inherent in the process. Likewise, a process under the influence of a common cause will always be stable and predictable.
Assignable Cause: It is also known as “special cause.” Therefore, the variation in a process that is not due to chance can be identified and eliminated. In this case, a process under the influence of a special cause will not be stable and predictable.
How to Perform SPC
1. identify the processes: .
Firstly, identify the key process that impacts the output of the product or the process that is very critical to the customer. For example, plate thickness impacts the product’s performance in a manufacturing company, then consider the plate manufacturing process.
2. Determine measurable attributes of the process:
Secondly, identify the attributes that need to be measured during production. For example, consider the plate thickness as a measurable attribute.
3. Determine the measurement method and also perform Gage R&R :
Thirdly, create a measurement method work instructions or procedure, including the measuring instrument. For example, consider a thickness gauge to measure the thickness and create an appropriate measuring procedure. Perform Gage Repeatability and Reproducibility (Gage R & R) to define the amount of variation in the measurement data due to the measurement system.
4. Develop a subgroup strategy and sampling plan:
Fourthly, determine the subgroup size based on the product’s criticality and determine the sampling size and frequency. For example, collect 20 sets of plate thicknesses in a time sequence with a subgroup size of 4.
5. Collect the data and plot the SPC chart:
Then, collect the data per sample size and select an appropriate SPC chart based on data type (Continuous or Discrete) and subgroup size. For Example, for plate thicknesses with a subgroup size of 4, select Xbar -R chart.
6. Describe the natural variation of attributes:
Next, calculate the control limits. From the above example, calculate the upper control limit (UCL) and lower control limit (LCL) for both Xbar Ranges .
7. Monitor process variation:
Finally, interpret the control chart and check whether any point is out of control and the pattern.
Example: Check the Xbar R chart if the process is not in control, then identify the assignable cause(s) and address the issue. This is an ongoing process, so monitor the process variation.
Additional Statistical Process Control Resources
Control limits are the voice of the process (different from specification limits , which are the customer’s voice.) They show what the process is doing and act as a guide for what it should be doing. Control limits also indicate that a process event or measurement is likely to fall within that limit.
Control charts : A Control chart is one of the primary statistical process control techniques (SPC). The control chart is a graphical display of quality characteristics that are measured or computed from a sample versus the sample number or time. Furthermore, the control chart contains a center line that represents the average value of the quality characteristics and two other horizontal lines known as the upper control limit (UCL) and lower control limit (LCL)
The selection of an appropriate control chart is very important in control chart mapping. Otherwise, it ends up with inaccurate control limits for the data. The control chart selection depends on the data type: Continuous or Discrete.
Variable (Continuous) Control Charts
Measure the output on a continuous scale. It is possible to measure the quality characteristics of a product.
X bar – R Charts (when data is readily available)
- X bar R chart is used to monitor the process performance of continuous data and the data to be collected in subgroups at set periods. In other words, two plots monitor the process mean and the process variation over time.
Run Charts (limited single-point data)
- A run chart displays observed data as they evolve over time. Just a basic graph that displays data values in a time order. It can be useful for identifying trends or shifts in the process but also allows you to test for randomness in the process.
X – MR Charts (I – MR, individual moving range)
- An I-MR chart is also known as an X-MR chart. An Individual moving range (I-MR ) chart is used when continuous data is not collected in subgroups. In other words, collect a single observation at a time. An I-MR chart provides process variation over time in a graphical method.
X bar – S Charts (when Sigma is readily available)
- X Bar S charts often used control charts to examine the process mean and standard deviation over time. These charts are used when the subgroups have large sample sizes, and the S chart provides a better understanding of the spread of subgroup data than the range.
- The EWMA – Exponentially Weighted Moving Average chart is used in statistical process control to monitor variables (or attributes that act like variables). Additionally, it makes use of the entire history of a given output. This differs from other control charts that treat each data point individually.
Attribute(Discrete) Control Charts :
The output is a decision or counting. It is not possible to measure the quality characteristics of a product. In other words, it is based on the visual inspection like good or bad, fail or pass, accept or reject.
- p Charts : (for defectives – sample size varies) – Use P chart when the data are the fraction defective of some set of the process output. It may also be shown as a percentage of defective. The points plotted on the p-chart are the fraction of non-confirming units or defective pieces found in n samples.
- np Charts (for defectives – sample size fixed) – Use an np-chart when the data is collected in subgroups that are the same size. Np-charts show how the process changes over time, measured by the number of nonconforming items (defectives) produced. In other words, the process describes pass or fail, yes or no.
- c Charts (for defects – sample size fixed) – Use c charts when the data are concerned with the number of defects in a product. The number of defects collected for the area of opportunity in each subgroup.
- u Charts (for defects – sample size varies) – A u chart is an attribute control chart that displays how the frequency of defects or nonconformities changes over time for a process or system. The number of defects collected for the area of opportunity in each subgroup.
Statistical Process Control Links
Great decision matrix here: https://www.moresteam.com/toolbox/statistical-process-control-spc.cfm
Statistical Process Control Videos
Six Sigma Green Belt Statistical Process Control Questions
Question: In the Control Phase of an LSS project, a Belt will identify key metrics that can be monitored and analyzed to give an indication that a process may be moving towards an out-of-spec condition. When he applies this approach, he is using __________________.
(A) Poisson Derivatives (B) Inferential Statistics (C) Kanban Analysis (D) Statistical Process Control
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Question: Statistical process control (SPC) is best defined as the use of
(A) Pareto charts to understand and control a process (B) inputs to control critical and complex processes (C) statistical methods to identify and remove manufacturing errors (D) statistical methods to understand and control a process
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How to conduct SPC FOR BATCH ORDER TYPE PRODUCTION?
Try making a control chart depending on what your quality measures are.
Could you direct me to the AIAG information you mentioned in the control phase quiz? I followed the link provided but do not see the answer. I tried searching in the search bar but no results.
Larry, you don’t really need the Automotive Industry Action Group (AIAG) information. By definition out of control could be considered a certain number of points in a row on one side of the X Bar Bar or R in a control chart or a certain number of points in a row that are consistently increasing or decreasing.
I’ve seen some standards list 5 points and others list 7. The key to the question is knowing that a process shift or trend has begun if you see that behavior emerged.
I’ve updated the answer walkthrough with this.
You can find AIAG’s PDF on Academia.com here: https://www.academia.edu/7829906/AIAG_Statistical_Process_Control_SPC_2nd_Edition
Note from Admin: This question has been moved to the private member’s forum here.
How to perform SPC for Angular Dimension??
I’ve never considered it, Ranjan. Perhaps this helps?
can u explain what the difference between SPC and control charts as it is mentioned that to check causes of variation in process we use SPC that is either from natural or random causes but then why we use control charts
Think of SPC as a framework and Control charts as a tool you would use within the framework.
Control charts give you an excellent visual representation of what is going on with your process.
hello when is this article published originally
My first publication was on June 27, 2014. There have been several revisions since that time.
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Introduction To Statistical Process Control
Home > Knowledge > Introduction To Statistical Process Control
If you work in the manufacturing industry, then you will know how important it is to check for defects as early as possible. SPC, or statistical process control is a set of methods that was first devised in the 1920s. W. Edwards Deming helped to standardize the idea of SPC during the Second World War, before introducing it to Japan after America’s occupation. SPC soon became a huge part of the Six-Sigma, and by extension of this, lean manufacturing .
Why is SPC Such a Useful Tool?
SPC essentially measures the overall output of a process. it works by exploring and documenting small, yet significant changes. This allows corrections to be made before any defects occur.
SPC was used originally in manufacturing, as it is one of the best ways to significantly reduce waste due to scrap utilization. Now, however, it is used in various service industries, as well as healthcare. SPC uses various statistical methods to try and monitor the output. SPC uses design experiments as well, as it is vital that the work is carried out in two different phases. The first phase would be to make sure that the process is fit for purpose, before establishing what it should look like.
The second phase monitors the process to make sure that everything is working as it should. Determining the correct frequency is so important, as it will in part, depend on various influences and significant factors. If you want to learn more about statistical process control or if you want to learn how to use control charts in terms of the statistical process, then keep on reading this introduction to statistical process control.
Key Concepts within Statistical Process Control
One of the key concepts within SPC is the variation within the process. This can be down to the two basic causes. If you look at Shewhart , you will soon see that these are documented quite clearly.
Assignable Causes and Chance Causes
A key concept in SPC would be assignable causes and chance causes. The basic idea is that if every process is constant, then some random variation is going to occur. This is unavoidable, but the great thing about statistical process control is that it can help us to understand it. By knowing that a process can only ever be impacted by chance, it becomes easier to calculate or predict the probability of a given part, being out of line in terms of general specification. In his introduction to statistical process control, Shewhart refers to other sources of variation, as being assignable causes.
Causes and Control Charts with Statistical Methods
These are not at all random in nature and are instead caused by identifiable events and changes.
It may be that another operator has taken over the manufacturing process, that the temperature has changed or that the materials being used in quality control have been altered. It is hard to predict what the output of any given process can be if you do not measure the assignable cause of the variation.
When you look at the modern implementation of this introduction to statistical process control guide, you will see that chance causes are referred to as being common causes. Assignable causes are called special causes.
Concepts parallel with MSA
Concepts like this, have parallels with MSA, or measurement system analysis. Common causes can be related to precision in MSA and assignable causes can be compared to trueness or bias.
In this introduction to statistical process control, we’re going to go into the significant special cause variations, and how they can be detected and removed quickly through control chart adoption.
Explanation through a Common Cause
One of the main aims of SPC is to try and achieve a process, where all of the variations can be explained through common causes.
This gives a known probability statistic in terms of defects. Shewhart once said that when something is controlled, it can be predicted.
You can predict the probability that the observed phenomenon is going to fall within the parameters during the manufacturing process. Industrial and service processes often rely on statistical methods such as this, as well as relying on control charts to ensure that everything is working as it should be.
In modern SPC, a process is stable when the variation appears but can be pinned down to a common cause. This is often done through control charts so that an expected level of variation can occur. Real processes might have a lot of different variations, but only a few of them are truly significant when you look at the control charts. During the initial phase of SPC, special causes are first identified and then removed in an attempt to stabilize the process. The limits can then be determined, but only if another special cause does not emerge.
Again, all of this can be documented through the use of control charts and through the process that statistical process control describes.
One example of this, which again, can be uncovered through the use of control charts, would be if a process that was once stable, begins to change. This could be as tooling wears for example. The overall concept of having a stable process can only be evaluated when any sources of bias have been eliminated.
By doing this through control charts, you are then left with a measurement that can only be influenced by known random influences. This is the best way to ensure a steady control chart, a reliable manufacturing process and fundamental multivariate SPC charts.
SPC is a huge subject, and it is entirely possible for it to involve some complex control charts and statistics.
Only a basic level of understanding regarding these control charts and statistics is required for you to control your manufacturing processes, however.
Some of the things you need to understand, in order to benefit quality control, would be standard deviation, statistical significance and probability distribution.
Standard deviation, in terms of control charts and quality control, is a measure of a set of values. If you have 20 parts in total at the end of a process, then you may find that each part has a very slight variation in terms of the measurement value. By importing this data into the control charts and by monitoring each control chart effectively, you can then gauge how much variation there is.
The simplest way for you to do this would be for you to first look at the smallest value and the largest value. Deduct them, and this will give you the range.
The issue here is that the more parts you check, the more range you’ll get, so it is impossible to determine the probability of general conformance with this range alone. The standard deviation would be a reliable measure, as it can be based on the assumption that the average distance for the values is taken from the mean.
When you consider dispersion, it is of no concern if the values are bigger or smaller than the mean. All that matters is how far away they are.
Squaring the difference, adding them together, and then dividing them will help you to get the mean. The probability with each score will increase from the lowest value to the middle value. It will then decrease to the largest value.
This probability distribution is called triangular distribution. If you look at control charts, or if you implement this within the control chart, then you will see that this happens when two random effects with a uniform distribution are added to give a combined effect.
When you combine random effects from the control chart, you will see that the point of the triangle eventually flattens, giving a Gaussian distribution. Normal distribution happens when lots of different effects, with different distributions, give a combined effect. This can be proven with the central limit theorem. This effect simply states that even in the incredibly complex system of the natural world, it is possible for some processes to be normal. Patient survival data analysis can also be incorporated here.
So ultimately, if we know the standard deviation of a process, it is possible to calculate the overall probability of an output given the range of values. The probability of any defect can be calculated with ease, as long as the value belongs to the distribution. If it is unlikely that the measured part could have come from the stable process then this indicates that a new, unknown special cause has emerged. This would then indicate that the process is out of control and that something has to be corrected so that the overall system can be brought back into line.
Understanding the process of control is essential if you want to maintain it properly. By taking the time to implement systems like this, it becomes very easy to document the whole process from start to finish. It also becomes easier to find any anomalies as soon as they occur, which is crucial in the world of manufacturing, as one small mistake that is not caught in time could have devastating effects on the whole production line.
Encyclopedia of Production and Manufacturing Management pp 50 Cite as
ASSIGNABLE CAUSES OF VARIATIONS
- Reference work entry
Assignable causes of variation are present in most production processes. These causes of variability are also called special causes of variation ( Deming, 1982 ). The sources of assignable variation can usually be identified (assigned to a specific cause) leading to their elimination. Tool wear, equipment that needs adjustment, defective materials, or operator error are typical sources of assignable variation. If assignable causes are present, the process cannot operate at its best. A process that is operating in the presence of assignable causes is said to be “out of statistical control.” Walter A. Shewhart (1931) suggested that assignable causes, or local sources of trouble, must be eliminated before managerial innovations leading to improved productivity can be achieved.
Assignable causes of variability can be detected leading to their correction through the use of control charts.
See Quality: The implications of W. Edwards Deming's approach ; Statistical process control ; Statistical...
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Deming, W. Edwards (1982). Out of the Crisis, Center for Advanced Engineering Study, Massachusetts Institute of Technology, Cambridge, Massachusetts.
Shewhart, W. A. (1939). Statistical Method from the Viewpoint of Quality Control, Graduate School, Department of Agriculture, Washington.
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(2000). ASSIGNABLE CAUSES OF VARIATIONS . In: Swamidass, P.M. (eds) Encyclopedia of Production and Manufacturing Management. Springer, Boston, MA . https://doi.org/10.1007/1-4020-0612-8_57
DOI : https://doi.org/10.1007/1-4020-0612-8_57
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Operations Management: An Integrated Approach, 5th Edition by
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SOURCES OF VARIATION: COMMON AND ASSIGNABLE CAUSES
If you look at bottles of a soft drink in a grocery store, you will notice that no two bottles are filled to exactly the same level. Some are filled slightly higher and some slightly lower. Similarly, if you look at blueberry muffins in a bakery, you will notice that some are slightly larger than others and some have more blueberries than others. These types of differences are completely normal. No two products are exactly alike because of slight differences in materials, workers, machines, tools, and other factors. These are called common , or random, causes of variation . Common causes of variation are based on random causes that we cannot identify. These types of variation are unavoidable and are due to slight differences in processing.
Random causes that cannot be identified.
An important task in quality control is to find out the range of natural random variation in a process. For example, if the average bottle of a soft drink called Cocoa Fizz contains 16 ounces of liquid, we may determine that the amount of natural variation is between 15.8 and 16.2 ounces. If this were the case, we would monitor the production process to make sure that the amount stays within this range. If production goes out of this range—bottles are found to contain on average 15.6 ounces—this would lead us to believe that there ...
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Module 8. Statistical quality control
BASIC CONCEPTS OF STATSITICAL QUALITY CONTROL
From the early days of industrial production, the emphasis had been on turning out products of uniform quality by ensuring use of similar raw materials, identical machines, and proper training of the operators. Inspite of these efforts, the causes of irregularity often crept in inadvertently. Besides, the men and machines are not infallible and give rise to the variation in the quality of the product. For keeping this variation within limits, in earlier days, the method used was 100 per cent inspection at various stages of manufacturing.
It was in 1924 that Dr. W.A. Shewhart of Bell Telephone Laboratories, USA developed a method based on statistical principles for controlling quality of products during the manufacturing and thus eliminating the need for 100 per cent inspection. This technique which is meant to be an integral part of any production process, does not provide an automatic corrective action but acts as sensor and signal for the variation in the quality. Therefore, the effectiveness of this method depends on the promptness with which a necessary corrective action is carried out on the process. This technique has since been developed by adding to its armory more and more charts, as a result of its extensive use in the industry during and after the Second World War. In this lesson various terms used in the context of Statistical Quality Control (SQC) have been illustrated.
26.2 Definitions of Various Terms Involved in Statistical Quality Control
The following terms are used to understand the concept of Statistical Quality Control
The most important word in the term ‘Statistical Quality Control’ is quality. By ‘Quality’ we mean an attribute of the product that determines its fitness for use. Quality can be further defined as “Composite product characteristics of engineering and manufacture that determine the degree to which the product in use will meet the expectations of the customer at reasonable cost.” Quality means conformity with certain prescribed standards in terms of size, weight, strength, colour , taste, package etc.
26.2.2 Quality characteristics
Quality of a product (or service) depends upon the various characteristics that a product possesses. For example, the Kulfi we buy should have the following characteristics.
(a) TS (b) Sugar (c) Flavour (d) Body & Texture.
All these individual characteristics constitute the quality of Kulfi . Of course, some of them are important (critical) without which the Kulfi is not acceptable. For example Minimum TS, Sugar, Body and Texture score is important. However, other characteristics such as Colour and Flavour may not be so important. The quality characteristics may be defined as the “distinguishing” factor of the product in the appearance, performance, length of life, dependability, reliability, durability, maintainability, taste, colour , usefulness etc. Control of these quality characteristics in turn means the control of the quality of product.
26.2.3 Types of characteristics
There are two types of characteristics viz., variable characteristics and attribute characteristics.
188.8.131.52 Variable characteristic
Whenever a record is made of an actual measured quality characteristic, such as dimension expressed in mm, cm etc. quality is said to be expressed by variables. This type of quality characteristics includes e.g., dimension (length, height, thickness etc.),hardness, temperature, tensile strength, weight, moisture percent, yield percent, fat percent etc.
184.108.40.206 Attribute characteristic
Whenever a record shows only the number of articles conforming and the number of articles failing to conform to any specified requirements, it is said to be a record of data by ‘attributes’. These include:
· Things judged by visual examination
· Conformance judged by gauges
· Number of defects in a given surface area etc.
Control means organizing the following steps:
· Setting up standards of performance.
· Comparing the actual observations against the standards.
· Taking corrective action whenever necessary.
· Modifying the standards if necessary.
26.2.5 Quality control
Quality control is a powerful productivity technique for effective diagnosis of lack of quality (or conformity to set standards) in any of the materials, processes, machines or end products. It is essential that the end products possess the qualities that the consumer expects of them, for the progress of the industry depends on the successful marketing of products. Quality control ensures this by insisting on quality specifications all along the line from the arrival of materials through each of their processing to the final delivery of goods.Quality control, therefore, covers all the factors and processes of production which may be broadly classified as follows:
· Quality of materials : Material of good quality will result in smooth processing there by reducing the waste and increasing the output. It will also give better finish to end products.
· Quality of manpower : Trained and qualified personnel will give increased efficiency due to the better quality production through the application of skill and also reduce production cost and waste.
· Quality of machines : Better quality equipment will result in efficient working due to lack or scarcity of break downs thus reducing the cost of defectives.
· Quality of Management : A good management is imperative for increase in efficiency, harmony in relations, growth of business and markets.
26.2.6 Chance and assignable causes of variation
Variation in the quality of the manufactured product in the repetitive process in the industry is inherent and inevitable. These variations are broadly classified as being due to two causes viz., ( i ) chance causes, and (ii) assignable causes.
220.127.116.11 Chance causes
Some “Stable pattern of variation” or “a constant cause system” is inherent in any particular scheme of production and inspection. This pattern results from many minor causes that behave in a random manner. The variation due to these causes is beyond the control of human being and cannot be prevented or eliminated under any circumstance. Such type of variation has got to be allowed within the stable pattern, usually termed as Allowable Variation. The range of such variation is known as natural tolerance of the process.
18.104.22.168 Assignable causes
The second type of variation attributed to any production process is due to non-random or the so called assignable causes and is termed as Preventable Variation. The assignable causes may creep in at any stage of the process, right from the arrival of raw materials to the final delivery of the goods.
Some of the important factors of assignable causes of variation are substandard or defective raw material, new techniques or operations, negligence of the operators, wrong or improper handling of machines, faulty equipment, unskilled or inexperienced technical staff and so on. These causes can be identified and eliminated and are to be discovered in a production process before it goes wrong i.e., before the production becomes defective.
26.3 Statistical Quality Control
By Statistical Quality Control (SQC) we mean the various statistical methods used for the maintenance of quality in a continuous flow of manufactured goods. The main purpose of SQC is to devise statistical techniques which help us in separating the assignable causes from chance causes of variation thus enabling us to take remedial action wherever assignable causes are present. The elimination of assignable causes of erratic fluctuations is described as bringing a process under control. A production process is said to be in a state of statistical control if it is governed by chance causes alone, in the absence of assignable causes of variation.
In the above problem, the main aim is to control the manufacturing process so that the proportion of defective items is not excessively large. This is known as ‘ Process Control’ . In another type of problem we want to ensure that lots of manufactured goods do not contain an excessively large proportion of defective items. This is known as ‘ Product or Lot Control ’. The process control and product control are two distinct problems, because even when the process is in control, so that the proportion of defective products for the entire output over a long period will not be large, an individual lot of items may not be of satisfactory quality. Process Control is achieved mainly through the technique of ‘ Control Charts ’ whereas Product Control is achieved through ‘ Sampling Inspection’ .
26.4 Stages of Production Process
Before production starts, a decision is necessary as to what is to be made. Next comes the actual manufacturing of the product. Finally it must be determined whether the product manufactured is what was intended. It is therefore necessary that quality of manufactured product may be looked at in terms of three functions of specification, production and inspection.
This tells us what is to be produced and of what specification. That is, it gives us dimension and limits within which dimension can vary. These specifications are laid down by the manufacturer.
Here we should look into what we have manufactured and what was intended to.
Here we examine with the help of SQC techniques whether the manufactured goods are within the specified limits or whether there is any necessity to widen the specifications or not. So SQC tells us as to what are the capabilities of the production process.
Therefore statistical quality control is considered as a kit of tools, which may influence decisions, related to the functions of specification, production or inspection. The effective use of SQC generally requires cooperation among those responsible for these three different functions or decisions at a higher level than any one of them. For this reason, the techniques should be understood at a management level that encompasses all the three functions.
Monday, August 17, 2015
Chance & assignable causes of variation.
Links to all courses Variation in quality of manufactured product in the respective process in industry is inherent & evitable. These variations are broadly classified as- i) Chance / Non assignable causes ii) Assignable causes i) Chance Causes: In any manufacturing process, it is not possible to produce goods of exactly the same quality. Variation is inevitable. Certain small variation is natural to the process, being due to chance causes and cannot be prevented. This variation is therefore called allowable . ii) Assignable Causes: This type of variation attributed to any production process is due to non-random or so called assignable causes and is termed as preventable variation . Assignable causes may creep in at any stage of the process, right from the arrival of the raw materials to the final delivery of goods. Some of the important factors of assignable causes of variation are - i) Substandard or defective raw materials ii) New techniques or operation iii) Negligence of the operators iv) Wrong or improper handling of machines v) Faulty equipment vi) Unskilled or inexperienced technical staff and so on. These causes can be identified and eliminated and are to discovered in a production process before the production becomes defective. SQC is a productivity enhancing & regulating technique ( PERT ) with three factors- i) Management ii) Methods iii) Mathematics Here, control is two-fold- controlling the process ( process control ) & controlling the finished products (products control).
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Quality Management System Glossary
Every manufacturing quality management professional who uses statistical process control (SPC) runs into questions occasionally. That’s why we’ve compiled this SPC glossary to serve as a quick reference when you’re looking for an answer, need to explain a concept to a colleague—or just can’t remember that term that’s on the tip of your tongue.
Feel free to bookmark this reference so you always have the definition you’re looking for—and be sure to visit our other SPC reference resources.
WHAT IS STATISTICAL PROCESS CONTROL? Learn the definition of SPC and what this industry-standard methodology is used for.
SPC 101 Dig in deeper to understand why and how SPC is used in manufacturing quality control.
DEFINITIVE GUIDE TO SPC CHARTS Learn why and how to use different control charts, see examples, and explore use cases.
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What is the variation?
Whatever measurement we take, there is always a variation between these measurements. No two items or measurements are precisely the same.
The problem with the variation is that it is the enemy of quality. Variation and quality do not go hand in hand. Variation reduction is one of the significant challenges of quality professionals.
Two types of variation, and why is it important to differentiate?
When dealing with variation, the challenge quality professionals face when to act and when not to act. Because if you act on each and every variation in the process and adjust the process, this will be a never-ending process. Dr. Deming called this "tempering the process." Rather than improving the quality, tempering, in fact, reduces the quality. Deming demonstrated the effect of tempering with the help of a funnel experiment.
The causes of variation can be classified into two categories:
- Common Causes
- Special Causes
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Common Cause Vs Special Cause: Types of Variation
Common cause variation is the natural variation in the process. It is a part of the process. There are "many" causes of this type of variation, and it is not easy to identify and remove these. You will need to live with them unless drastic action is taken, such as process re-engineering.
Common causes are also called n atural causes, noise, non-assignable and random causes .
Special cause variation, on the other hand, is the unexpected variation in the process. There is a specific cause that can be assigned to the variation. For that reason, this is also called as the assignable cause . You are required to take action to address these variations.
Special causes are also called assignable causes .
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If the measurements of a process are normally distributed, then there is a 99.73% chance that the measurement will be within plus and minus three standard deviations. This is the basis of control charts .
If you plot the measurements on a Control Chart, then any measurements which are outside the plus and minus three standard deviation limits are expected to be because of a special cause. These limits are called as the Upper Control Limit (UCL) and the Lower Control Limits (LCL), Once you get such measurement, you are expected to investigate, do the root cause analysis , find out the reason for such deviation and take necessary actions.
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Statistical Process Control (SPC)
In our daily life, we utilize a variety of products and services from different outlets. We use products such as mobile, electrical bulbs, clothes, etc. and use different types of services such as health care, transportation, consulting, etc. All these services and products should attain certain specifications when we use it, whether it can be good or bad. We are on the tough competitive world and so the main aim of the manufacturer or provider is to achieve quality assurance where it can meet the customer expectations.
In such situations, we require a tool or technique through which we can control the process. This technique is known as statistical process control. For understanding SPC, first of all, we should understand the concept of process in quality control. A process is a series of operations or actions that transforms input to output.
What is Statistical Process Control?
SPC is a method which is used for understanding and monitoring the process by collecting data on quality characteristics periodically from the process, analyzing them and taking suitable actions whenever there is a difference between actual quality and the specifications or standard. It is a decision-making tool and widely used in almost all manufacturing processes for achieving process stability to continuous improvements in product quality.
Brief History of the Origin of SPC
During the 1920s , Walter A. Shewhart discovered a way to distinguish between common and special causes of variation in a process. This lead to an invention of the widely known method as statistical process control (SPC) . He pioneered the use of statistical techniques for monitoring and controlling quality. Bell Labs wanted to economically monitor and control the variation in the quality of components and finished products. He recognized that inspecting and rejecting or reworking product was not the most economical way to produce a high-quality product. He demonstrated that monitoring and controlling variation throughout production was the more efficient and economical way.
Shewhart developed a visual tool for monitoring process variation, which came to be known as the control chart or the Shewhart control chart and the concept of a state of statistical control in 1924 at Bell Laboratories.
He has defined chance and assignable causes as the two sources of quality variation. A process that is operating with the only chance cause of variation is said to be in statistical control. A process that is operating with the assignable cause of variation is said to be out of control. The underlying concept of the Shewhart chart is to construct its limits based on variation allowable as it is in – control state and monitor the quality of the product produced.
Bell Labs was widely recognized as the ‘international standard for quality’ by the 1930s, due to the large applications of Shewhart’s techniques in the field of telecommunications. During this period, many initiatives were done by conducting extraordinary research in statistical methods to control and improve process variation. This leads to improve product quality in a great way.
His work was summarized in his books titled “Economic Control of Quality of Manufactured Product” (1931) and “Statistical Method from the Viewpoint of Quality Control” (1939) .
Image – Referred from leansixsigmadefinition.com
Breakthrough in the evolution of SPC
SPC was pioneered by Walter A. Shewhart in the 1920s. W. Edwards Deming applied SPC methods in the United States during World War II, to improve quality in the manufacture of weapons and other important products needed during the war period. After the devastating defeat of Japan in World War II, the United States led the Allies in the occupation and rehabilitation of the Japanese state. In 1947, Deming was involved in early planning for the 1951 Japanese Census. The Allied powers were occupying Japan, and he was asked by the United States Department of the Army to assist with the census. Deming was also instrumental in introducing SPC methods to Japanese industry after the war had ended.
Deming’s mastered the Shewhart’s ideas by implementing it to Japanese industry from 1950 onwards. He developed and added some of his techniques to Shewhart’s methodology. Later he named as the ‘Shewhart cycle’ . Deming’s approach to quality management results in continuous improvement of the production process to achieve conformance to specifications and reduce variability. He identifies two primary sources of process improvement: eliminating common causes of quality problems, such as poor product design and insufficient employee training, and eliminating special causes, such as specific equipment or an operator.
He was widely known for his work contribution for Japanese industry and for the new development era. He received an invitation from the Japanese Union of Scientists and Engineers (JUSE) and worked as an expert to teach statistical control. He trained hundreds of engineers, managers, and scholars in SPC and concepts of quality.
During the 1960s and 1970s, SPC grew rapidly in Japan and was a successful in quality improvement goals. Later other countries started implementing SPC in their process.
His work was summarized in his books titled “ Out of the Crisis” (1982 – 1986) and “ The New Economics for Industry, Government, Education” (1993) , which includes his System of Profound Knowledge and the 14 Points for Management.
Image – Referred from census.gov
Statistical process control also termed as SPC is a statistical method used to monitor, control and improve processes by eliminating variation from industrial, actuarial, service and many other processes. Here we can determine if an improvement is actually happening and also use them to predict whether it is statistically capable to meet the specific target or not. The main aim of using SPC is to understand where the focus of works needs to be done in order to make a difference. It has now been incorporated by organizations around the world as a primary tool to improve product quality by reducing process variation.
During the initial phase, the SPC was applied only on manufacturing industries for quality improvement and so on. As time evolves by, it was started applying on service industry such as airlines, hospitals, insurance companies, etc. Now on this advanced age of science and information technology, it has started applying on big data analytics to artificial intelligence and much more to advance.
SPC involves following phases of activity –
- Collection of data from a process.
- Identification of causes and to eliminate it.
- Track process variation.
- Diagnosing the deviated process.
- Implementing corrective action.
(We use basic quality tools on these phases)
How many Types of variation are there in a process?
SPC is implemented in industry to detect a process variation and to eliminate it for better quality assurance. By monitoring the performance of a process, we can detect trends or changes in the process before they produce non-conforming product and scrap. [By reducing variation]
Variation can be divided as common cause variation and special cause variation.
- Chance causes are also known as random or natural or common causes . It is due to the natural variation of the process; i.e. Variation due to the way the process was designed and we cannot identify. For example, the fuel efficiency of machine varies slightly; the diameter of a bottle cap varies slightly and so on. (Statistically in control)
- Assignable causes are also known as special or non-random or unnatural causes. Causes can be identified and eliminated – poor employee training, equipment nonfunctional, etc. An example of special cause variation is the variation that might result if someone untrained is allowed to work in the process. (Out of control)
When to use SPC?
- To have an overall glimpse of a process.
- Monitoring a process to check whether it is under control or out of control.
- To track variation and to eliminate it from a process.
- Improvement in process capability aspects.
- To increase production by reducing scrap, rework and inspection cost.
What are the benefits of SPC?
- Early detection of variation in a process.
- Establish a consistent level of quality.
- Continuous improvement in a process by reducing variation.
- Helps in decision making by giving the insights of process.
- Reduce or eliminate the need for inspection during the supply chain.
- Lower investment because of process improvements.
- It provides real time analysis of a process and so we can focus on areas needed for improvement.
- Efficiency in data entry, analysis and reporting.
What is Process capability analysis?
It is one of the primary tools in SPC. Suppose in a manufacturing process or any process, we often required information about the process w.r.t its performance or capability. Basically, it refers to the capability of a process to meet customer requirements or industrial standards on a consistent basis.
Measures of Process Capability – Process capability can be measured by the following methods.
- Process capability Ratio (C p ) – It is often described as the capability of a process when the process data is centred and specification limits are known.
USL -> Upper specification limit
LSL -> Lower specification limit
σ -> Process standard deviation
- Process capability index (Cpk) – It is described as the capability of a process when the process data is not centered and only one of the specification limits are known.
Some important considerations
- When Cp=Cpk –> process is centered at the midpoint of specifications.
- When Cp>Cpk –> process mean is nearer to one specification limit or the other.
- When Cp< 0 –> process mean lies outside the limit.
In a piston manufacturing industry, quality engineers want to assess the process capability. They collect 25 subgroups of five piston rings and measure the diameters. The specification limits for piston ring diameter are 74.0 mm ± 0.05 mm.
All the measurements are within the specification limits. The process is on target and the measurements are approximately centred between the specification limits.
What are Control limits and Specification limits?
Control limits – Control limits describe the behaviour of a process which operates in a normal condition. It is basically a horizontal lines drawn on a control chart to examines the outlook of a process. It consists of UCL (Upper control limit), CL (Control limit) and LCL (Lower control limit). If the points lie beyond the limits, then there is an occurrence of a special cause of variation and henceforth.
Specification limits – Specification limits are the values on which the process should give a response within the range. It is based on customer requirements. It can be a plot in a histogram and consists of USL (Upper specification limit) and LSL (Lower specification limit).
Control limits reflect the real capability of a process whereas specification limits reflect the requirement of a customer. A process under control may not deliver the products under the given specifications.
What are the Challenges we face while implementing SPC?
In this competitive world, every industry wants to be better than others and to achieve the highest level of success. Suppose it can be in the quality field, continuous improvement in a process, efficient productivity and so on. To achieve this level of success, SPC plays an enormous role in a company and there are some of the challenges one may face while implementing SPC.
Some of them are:-
- Lack of effective training – Training is an important factor for the successful implementation of SPC. Proper training should be given to all the employees who work on a ground level to topmost level in a process.
- Lack of basic statistics knowledge – One should have basic knowledge about statistics. So they can relate the background of SPC methods. Suppose if they were using the histogram in a process then they should have basic knowledge about it.
- Responsibilities should be properly defined – Starting from operators to engineers; everyone should have a clear picture of their responsibility in a process. Engineers should have the basic statistical concepts in SPC whereas operators should be good in measurement and plotting it.
- Management immense support – Elite members of a company should encourage all the employees in all the levels. Management should give time to implement SPC in a proper way. They should never carry away with the time and cost it took to implement it. And nevertheless, wait for its result. Hard work always paid off.
Where can we apply Statistical Process Control?
Some of them are discussed below.
- DMAIC – It is a well-known Six Sigma methodology and focused on improving the process. DMAIC stands for Define Measure Analyze Improve and Control. SPC is widely used in Measure, Analyze and Control phases. During the Measure phase, it is used to set the process baseline by doing control chart analysis and Capability analyses are done to check the capability of a process to meet specifications. During the Control phase, it is used to monitor and improve the process.
To enhance success in lean manufacturing , six sigma and lean six sigma projects, SPC has to be properly used. Apart from these scenarios, we can use SPC tool individually to check the process capability for continuous improvement. Also, prove useful while conducting DOE in a process.
SPC in Total Quality Management (TQM)
In this competitive world, every industry has to compete with each other in terms of quality, production, revenue and so on. The main terminology which satisfies customer needs is “ quality ” which defines the company standard and values.
Within an organization, when TQM has implemented it helps for continuous improvement of process and gives consistently high-quality products. Total Quality Management is defined by the Deming Prize Committee as
- set of systematic activities
- carried out by the entire organization to effectively and efficiently
- achieve the organization’s objectives
- so as to provide products and services
- with a level of quality
- that satisfies customers ,
- at the appropriate time and price.
Statistical process technique (SPC) is a method used in TQM framework for detecting and reducing variation in a process. It is a very powerful method to detect, control, analyze and improve the process by reducing the source of variation. Hence SPC contributes a lot in TQM goal of continuous improvements.
What kind of Organisations can benefit with SPC?
Statistical process control also termed as SPC is a statistical method used to monitor, control and improve processes by eliminating variation from industrial, actuarial, service and many other processes. When an organization first uses SPC, the main objective is to ensure that the process is stable and capable of producing product or services to the expectations. It is widely known as a decision-making tool.
During the initial phase of SPC, it was used in discrete manufacturing (Telecom, Defense, Automobiles, etc.) and later it was applied to process manufacturing (Glass, Pharmaceutical, Beverage, etc.) too. It is widely used in almost all manufacturing processes for achieving process stability to continuous improvements in product quality.
But in recent years, SPC has implemented in various service sectors like healthcare, financial institutions, call centres, hotels, etc. The service industry has been an integral part of our life. They offer services which are very essential to us – starting from health care, airlines, call centres, banks and so on. For e.g. we often travel to various destinations for official work on holidays by air and stay in a hotel. When we travel by particular airline and didn’t get the essential services – Will we travel again from that airline? Similarly, when we stay at a particular hotel and didn’t get the required services – Will we stay again at that hotel? Our answer will be no, never. So maintaining healthy growth and improving the service quality will have significant impacts on us. And also excellent service quality is noted as a major factor to make a profit in the service sector.
Some of the examples are:-
- Healthcare – While implementing SPC we can improve patient care by reducing waiting time, and monitoring clinical trials, operational performance and so on.
- Banking – While implementing SPC we can improve customer service by reducing waiting time, % errors in customer profile, etc.
- Customer service – While implementing SPC we can improve customer service by reducing the call waiting time, monitoring the response calls, identification of a process whether it is under time limit or not, etc.
In a can-filling process, the quality engineer wants to know whether the process is in control or not. Each hour, they collect a subgroup of 10 cans. To minimize the variation (within subgroup), they collect the cans for a given subgroup in a short period of time.
They create an X-bar chart to monitor the weight of the cans.
With reference from the X-bar chart, one point is out-of-control and they conclude that the process is not stable. Hence the process should be improved.
Attend our Training Program, to know more about Statistics and Statistical Software. We conduct various training programs – Statistical Training and Minitab Software Training. Some of the Statistical training certified courses are Predictive Analytics Masterclass, Essential Statistics For Business Analytics, SPC Masterclass, DOE Masterclass, etc. (Basic to Advanced Level). Some of the Minitab software training certified courses are Minitab Essentials, Statistical Tools for Pharmaceuticals, Statistical Quality Analysis & Factorial Designs, etc. (Basic to Advanced Level).
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US Supreme Court rejects ex-cop Chauvin's appeal in George Floyd murder
WASHINGTON, Nov 20 (Reuters) - The U.S. Supreme Court on Monday declined to hear an appeal by former Minneapolis police officer Derek Chauvin of his conviction for the murder of George Floyd during a 2020 arrest, which sparked widespread protests against police brutality and racism.
The justices turned away Chauvin's appeal that he filed after a Minnesota appellate court upheld his 2021 murder conviction and rejected his request for a new trial. Chauvin had argued that jury bias and certain rulings by the presiding judge deprived him of his right to a fair trial under the U.S. Constitution's Sixth Amendment.
Chauvin, who is white, is serving a 22-1/2 year prison sentence for murdering Floyd, who was Black, by kneeling on a handcuffed Floyd's neck for more than nine minutes during an arrest.
Floyd's murder triggered protests in many cities in the United States and in other countries and focused attention on the issue of racial justice.
Chauvin, now 47, was found guilty by a 12-member jury in April 2021 of three charges of second-degree murder, third-degree murder and manslaughter following a three-week trial that included testimony from 45 witnesses, including bystanders, police officials and medical experts.
The guilty verdict marked a milestone in the fraught racial history of the United States and a rebuke of law enforcement's treatment of Black Americans.
In a May 25, 2020, confrontation captured on video by onlookers, Chauvin pushed his knee into the neck of Floyd, 46, while Chauvin and three fellow officers were attempting to arrest Floyd, who was accused of using a fake $20 bill to buy cigarettes at a grocery store.
[1/2] Community members visit one of the murals at George Floyd Square, now behind barricades that formerly blocked the street, after city employees began to reopen George Floyd Square, the area where George Floyd was killed in Minneapolis police custody the year before, in Minneapolis, Minnesota, U.S.... Acquire Licensing Rights Read more
On appeal in 2022, Chauvin's lawyer, William Mohrman, argued that Hennepin County District Judge Peter Cahill made multiple errors.
Because of extensive pre-trial publicity, the judge should have agreed to Chauvin's motions to move the trial outside Minneapolis and sequester the jury, Mohrman argued.
The judge, Chauvin's attorneys and prosecutors spent about two weeks questioning potential jurors before seating the 12-member panel.
Chauvin's attorney urged the Supreme Court to grant the appeal to consider whether jurors had been biased by a desire to avoid the "threat of harm to the community if a guilty verdict was not reached." His attorney also said one juror may have concealed possible bias by failing to disclose during the jury selection process that he had attended "an anti-police 'George Floyd' rally."
Attorneys for Minnesota did not respond to Chauvin's petition asking the Supreme Court to hear his appeal.
The Minnesota Court of Appeals in April rebuffed Chauvin's appeal, upholding his conviction and rejecting his request for a new trial. Minnesota's top court in July denied Chauvin's request to review the case, prompting his appeal to the U.S. Supreme Court.
Separately, Chauvin in December 2021 pleaded guilty in federal court to charges that he violated Floyd's civil rights. Chauvin on Nov. 13 filed a motion seeking to overturn that conviction based on what he claims is new evidence showing that Floyd's death resulted from an underlying medical condition.
Reporting by John Kruzel; Editing by Will Dunham
Our Standards: The Thomson Reuters Trust Principles.
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