I’m looking into control charts and realized that the way we calculate control limits for x bar is different when using R chart and S chart. When using the same set of data, the control limits are slightly different from others. Why does this happen and how should we understand the differences?
Remember that R chart hinges on the fact that they are using d2. Meanwhile, S chart hinges on c4. R chart and S chart are similar in application. The main difference between the two is their usage. R chart is generally good for small sample sizes. That being said, they are simpler to compute by hand. On the contrary, S chart is more responsive to variable sample sizes, especially when the sample size is large.