Calculation detail for X-bar/S Control Charts

1. Calculate average and standard deviation for each subgroup in the standard manner (as a normal standard deviation calculation).

2. Calculate the upper and lower control limits. This again requires a magic constant to be used, based on the number of measures in each subgroup. A table for this is included in Fig. 3.
   Note: the X-bar chart is calculated in exactly the same way as for the average and range charts.

Calculating X-bar/S control limits 

Next time: Calculation detail for defective (p) Control Charts

This article first appeared in Quality World, the journal of the Chartered Quality Institute