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Control Chart


Control Chart is a graphical tool used to determine process performance with time. These were developed by Walter A. Shewhart and hence are also known as Shewhart charts. These charts help determine whether the process is stable or not. If the process is stable, the process will only have inherent variation (Common Cause variation). The region of common cause variation is given by Control limits (Upper and Lower control limits). Any process variation outside these limits is considered as Special Cause Variation.


Mistake Proofing


Mistake Proofing or Poka Yoke - is a technique for eliminating errors by making it impossible to make mistakes in the process. It is often considered the best approach to process control.

It works on the principle - "It is good to do it right the first time. It is even better to make it impossible to do it wrong the first time".

Mistake Proofing can be preventive (prevent the error from occurring) or detective (detect the error if it happens) in nature and is usually implemented in one of the following ways.

1. Warning: audio and/or visual warning either before the error occurs or after it has occurred.
2. Control: errors are not possible or even if an error occurs, it is not allowed to move to the next step.
3. Shutdown: machine or process shutdown when an error is about to happen or as soon as an error occurs.


An application-oriented question on the topic along with responses can be seen below. The best answer was provided by Prashanth Datta on 22nd February 2019.


Applause for all the respondents- Prashanth Datta, Tarun Gupta


Also review the answer provided by Mr Venugopal R, Benchmark Six Sigma's in-house expert.



Q. 137  There are different ways and means of controlling a process and they vary in their effectiveness. Describe different type of process controls and sort them in your order of preference from best to worst.  


Note for website visitors - Two questions are asked every week on this platform. One on Tuesday and the other on Friday.


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Control Phase is that critical stage of Business Process Improvement where the selected solution(s) which are implemented to achieve the desired output [Y] are monitored for it's effectiveness. In other words, the performance of the process which needs to be maintained at the desired level as per the Voice of Business or Voice of Customer has to be sustained through the actions implemented. 

An effective Control Plan
a. Acts as Risk Assessment mechanism on the new improved process.
b. Ensures process stays within control and helps identify any out of control variations due to any special causes and calls for appropriate action as required
c. Continues to be a living document to monitor and control the new process
d. Doesn't replace the Standard Operation Procedure derived during the improved phase but adds to its effectiveness by monitoring it.
e. Resides with the process owner for continuous review and documentation.


Now knowing the importance of Control phase, let us look at some of the types of Process Controls and their effectiveness. I have kind of arranged these controls in order of their effectiveness basis my understanding and judgement

1. Mistake Proofing - Poke Yoke
2. Risk Mitigation Methodologies like FMEA
3. Process documentation / Inspection and Audits
4. Statistical Process Controls
5. Response and Reaction Plans
6. Process Ownership


1.Mistake Proofing - Poke Yoke - Mistake Proofing is a technique wherein the input's or causes are so well controlled by making it impossible to for a mistake to happen at the process level itself.

A simplest example that happens in our day to day life is using a spell check on our email. Unless all spellings are checked and corrected, the controls implemented in the form of a dialog box will not allow the email to be sent.

2. Risk Mitigation Methods like FMEA (Failure Mode and Effects Analysis), wherein every solution implemented to drive the Process Documentation / Inspection and Audits - I given an equal weightage to both these controls as an inspection or audit is done against a set standard and that standard has to be well documented in your process documentation. 

A well documented Standard Operating Procedure provides detailed instructions at every stage and what needs to be done at each stage which implies the required controls for possible deviations would also have been included in the document. 

While Inspection and Audits are an additional layer in the system which can add some additional lead time, it is still a desired quality control which helps to ensure the end results are controlled internally rather than treating as an escalation from customers.

3. Statistical Process Controls, like control charts helps as visual aids and paves way for analytics. It helps determine if a process is stable and within statistical controls. It serves as good trigger points to catch and deviation, however further deep dive has to be conducted to understand the causes for save.

4. Response and Reaction Plans - As suggested, the plans are reactive which is a response to an issue i.e. it comes into effect once an issue is identified. While it is not a top preferred option, but still is a control plan as it sets directions to have a response plan in the event of an issue trigger.

5. Process Ownership - Typically a control which is people dependent i.e. the process owner will define the rules to control the process outcome.

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                      Benchmark Six Sigma Expert View by Venugopal R


So long as we use the term “Control”, it implies that we expect variations from the process, and that we do not have perfect confidence in the process. Come to think about it, do we have any process in our day to day life that we can imagine without any controls?


One simple example that often crosses my mind is “Can we ride a bicycle if the handle bar is kept straight and locked?” You may try it out! Every moment we are riding the bicycle the control on balance is provided by the small movement of the handle bar, even if we want to ride straight! We walk straight on a path because, there is a control mechanism through our eyes that keeps directing every step, with some reference.


If we think carefully about any process, there are bound to be some built-in controls that make the process work the way it is intended.


I am not going to discuss any preferred order about the type of controls since most of the excellence ambassadors will have their experienced views. I am limiting my discussion on some aspects relating to effectiveness of controls.


Most of you will agree, controls may be broadly classified as Preventive, Detective and Corrective. Controls may also be classified as ‘Technical’ and ‘Statistical’.


What could be factors that adversely impact the effectiveness of these controls? A few of my thoughts are as below:


1. Missing out in the ‘anticipatory list

If a failure has not even been anticipated, (maybe it was missed out despite doing FMEA / maybe there was no history of past occurrence), then obviously no control will exist if such a failure occurs. The exhaustiveness of the list of anticipated failures is a factor and there is always the question “How do I know what I don’t know?”


2. Lack of control on the “Preventive Control”

Once we term a control as “Preventive”, can we sit back trusting that nothing can go wrong? Every “Preventive control” will have to be assessed for its effectiveness from time to time. For example, a loan disbursement will not be allowed by the system, unless the security clearances are fulfilled. Or an elevator should not start if it is overloaded. Such controls will have to be identified and evaluated from time to time to pro-actively ensure their functioning.


3. Misuse of Preventive / Detection controls

This is about having the knowledge of such controls and manipulating them. For example, disabling the Firewall system deliberately to allow certain downloads. Tampering with a smoke detection system (may be to enjoy a smoke!)


4. Not paying heed to the ‘warning’ based controls

This could happen either due to lack of awareness and training, or ‘taking for granted’ syndrome, or missing out on the warning.

If a ‘low oil’ symbol gets illuminated on your car dashboard, the driver should be educated to understand it and take action. (awareness)

If a fire alarm sounds, people tend to take it as a false alarm and not rush out (taking for granted)

During a vendor payment, an alert message pops up on the screen to verify if an abnormally high amount appears, but if it escapes the attention of the processor, an over payment might occur (missing out).


I am sure that we can think of more factors and examples that influence the effectiveness of process controls. Thus, the pursuit for perfecting a process control system needs to be a continual effort and there would always be a room for improving.

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Poka-Yoke or “mistake proofing” at the system level helps “prevent” the unintentional errors of commission. It is in my view the best control method because of its preventive approach to defect reduction as opposed to other reactive or responsive approaches, where errors are allowed to occur and then investment is needed for detecting them and eventually correct them, albeit all of that happens post facto.

Poka-Yoka has to be the most effective in control method owing to its zero-defect approach with 100% inspection, and fairly low cost deployment, as once the checks/validations are designed in the system itself, there is zero to little need for any warning systems as well any human checking efforts.


Statistical Process Control OR Control Charts provides for visibility into the process variation, monitoring if the process is operating as expected, or if at any point a corrective action is required. A process is said to be in stable state when only the common cause variations (those within the control limits) remain, while all the special cause variations (those outside the control limits) have been eliminated.

A stable process though may still not be meeting the customer specifications, and so would be deemed as stable yet not capable process, and needs improvements or a redesign. A rather reactive but still a potent tool to detect when to intervene for improvements or plugging the gaps.


CBA or Cost Benefit Analysis is the most pre-emptive strike on mitigating the risk of financial loss, as it is designed nip the bad project in the bud. It exposes the projects benefits in quantifiable terms – money needed, time involved and the net gains. It enables the project lead to decide that if the organization strongly desires the project despite CBA not showing favorable results, then what alterations can be made to the design and costs.

CBA is mostly beneficial when done it proactively before initiating the project, whereas most of the process improvement initiatives are taken up when the process is already in a state of concern.

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The chosen best answer is that of Prashanth as it lists most often used control methods, though I would change the order slightly:


1 hour ago, Prashanth Datta said:

1. Mistake Proofing - Poke Yoke
2. Risk Mitigation Methodologies like FMEA
3. Process documentation / Inspection and Audits
4. Statistical Process Controls
5. Response and Reaction Plans
6. Process Ownership


As mentioned by Venugopal, risk studies such a FMEA are the starting point to instituting any controls. Of course, FMEA is also repeated at some frequency and that can be considered as a control, too. I would re-order the list as:


1. Mistake Proofing - Poke Yoke
2. Statistical Process Controls

3. Response and Reaction Plans - this assuming that the error is being detected and corrected before it becomes a defect.

4. Inspection and Audits
5. Process documentation
6. Process Ownership

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