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Control Charts - Sample Size

Vishwadeep Khatri
Message added by Mayank Gupta,

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.


Sample Size (Subgroup Size) in a control chart is the number of data points collected in a given time period. 


An application-oriented question on the topic along with responses can be seen below. The best answer was provided by Sachin Tanwar on 2nd Apr 2024.


Applause for all the respondents - Sachin Tanwar.


Q 656An important input while working with control charts is to decide the sample size for monitoring. For e.g. - we could pick a sample of 1 item, 5 items, 20 items etc. How is this sample size decided? What happens if sample size is too low or too high?


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Sample Size in Control Charts: 
Sample size for control charts is chosen based on factors like process variation, stability, and resource constraints.

If the sample size is too low:

  • It may miss important process changes
  • Estimates of process variation may be unreliable

If the sample size is too high:

  • It can be costly and time-consuming
  • The chart may overreact to small changes, leading to not meaningful variations

Finding the right balance of samples [variable] and range [Time frame] is key for effective monitoring and control. 

Attaching my practice control chart image, where the sample count [ Week on week Quality Scores ] was enough to determine my process health. 

Control Chart.png

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