When you collect data in subgroups, you can use any of the following to estimate your process variation:
- Sbar is the average of the subgroup standard deviations. By default, if you create an S chart or an Xbar-S chart, Minitab uses Sbar to estimate the standard deviation of your process.
- Rbar is the mean of the subgroup ranges. By default, if you create an R chart or an Xbar-R chart, Minitab uses Rbar to estimate the standard deviation of your process.
- Pooled standard deviation
- The pooled standard deviation is a weighted average of the subgroup standard deviations. The weighting gives larger groups a proportionally greater influence on the overall estimate. Minitab uses the pooled standard deviation for capability analysis and for some control charts.