Within standard deviation is an estimate of the variation within the subgroups. If your data are collected properly, the within-subgroup variation should not be influenced by changes to process inputs, such as tool wear or different lots of material. In that case, the within standard deviation represents the natural and inherent variation of the process over a short period of time. It indicates the potential variation of the process if shifts and drifts between subgroups were eliminated.
If you use a data transformation when you perform the capability analysis, Minitab also calculates StDev(Within)*, the within subgroup standard deviation of the transformed data.
Larger values of within-subgroup standard deviation indicate greater variation with the subgroups. For example, if each subgroup is collected from a different batch of items, large within-subgroup standard deviation indicates a large amount of variability among the items within each batch.
Minitab uses the within-subgroup standard deviation to calculate the between/within standard deviation.