Which of the control charts must have at least 20 subgroups of observed values to judge whether a process is in control or not?

Question

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An R-chart must have at least 20 subgroups of observed values to judge whether a process is in control or not.
Key Takeaway: An R-chart contains data collected from a business or industrial process at regular intervals. It measures the sample range over time which is a measure of dispersion. An R-chart must have at least 20 subgroups of observed values with each subgroup containing 3 to 6 observations. This minimum number of observations allows a manager to observe whether the variation falls within the upper and lower control limits.

Which of the Control Charts Must Have at Least 20