Q 613. "Control Limits cannot be decided but they can be influenced". Explain this statement along with examples.

 

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The phrase "Control Limits cannot be decided but they can be influenced" refers to the process monitoring, and control method known as statistical process control, or SPC. The boundaries that specify the permissible variance in a process are known as control limits. They are developed using process capability analysis and historical data.

Here's an explanation with illustrations:

Control Limits Are Indeterminate:

Control limits are not just any old number that may be chosen whenever you want. The process's inherent unpredictability determines them. The control limitations will be broader for processes that are extremely variable and intrinsically unstable. On the other hand, if the process has low variability and stability, the control limits will be smaller.

As an illustration, think of a factory that makes lightbulbs. If the raw materials, machinery, and environmental circumstances remain constant, there will be minimal variation in the light bulb's output, such as wattage and lifespan. The regulatory restrictions will be strict in this situation. But the variability will rise, and the control limits will get larger if raw materials or machine settings are changed frequently.

Control Limits Are Subject to Change:

Although the intrinsic variability of the process plays a major role in determining control limits, modifications to the process or shifts in influencing factors can also have an impact.

As an illustration, suppose that the same light bulb manufacturer introduces a new quality control system that lowers process variability. Consequently, the light bulb's output becomes less variable. This may result in fewer control boundaries. In this instance, process advancements have affected the control boundaries. An additional instance could be the implementation of a more accurate production apparatus, this might make the output less variable, enabling tighter control limits. To sum up, process intrinsic unpredictability is the source of control constraints. They must be chosen based on process analysis and historical data; they are not arbitrary. On the other hand, they can be affected by process enhancements that lower variability. This illustrates that although control limitations cannot be imposed arbitrarily, they can be somewhat controlled by process enhancements.

control limits are the way of looking how the process is performing. weather the process variation is within the control limit or not.

ucl-upper control limit-this is the max variation from centerline a process can go.

lcl-lower control limit-This is the lowest variation a from centerline a process can go.

If any point of time the process goes beyond ucl or below lcl the process is consider to be unstable.

Control limit essentially determines whether our processes are in control or out of control.

 

To a large extent the control limits are indluenced because these are data driven.

 

I did a process on Machine Learning where the control was 5% was acceptable. So basically 95% was agreed confidence of identifying 95 high risk people out of 100.

 

Today in a volume of 200 , we say 5 % we can miss, which is a tolerance or control, maining 10 high risk people ot type B can be missed.

 

Suddenly the volume dipped to 50 customers the next working day and we still missed 6 high risk people meaning 12 % loss instead of 5 %. The places and situations where whole number needs to be applied are greatly influenced.

 

This meant that although our control limit was 90 % lower and 95 % higher, we still were 88 percent which influenced the situation. And the influencer was none other than the data.

 

The concept of control limits in statistical process control says that control limits are not arbitrarily determined but are instead based on data and statistical analysis. These limits are initially set to reflect the process's historical performance, and they represent the typical variation in the process under stable conditions. Control limits can be influenced or adjusted when process improvements or changes occur. This practice ensures that control charts remain relevant and effective in monitoring the process's ongoing performance and aligning with the principles of continuous improvement.
 

Example

In a roulette game in Casino, the control limits can be set by management based on roulette wheel design. In case the Casino notices an unusual pattern, they can influence the control limits by recalibration / changing to a new wheel. This proves that control limits are not rigid but can be adjusted to maintain the integrity of the game

Glad that most respondents answered correctly. The best answer has been given by Keerthi Vasan. Well done!