Calculation detail for X-MR Control Charts

Control Charts, as described in previous articles, are used to detect statistically-significant variation in processes and hence enables variation and quality to be pro-actively controlled. There are seven types of control chart, each of which has a different calculation for the upper and lower control limits. This month, we are covering the calculations for variables charts. 

1. Calculate basic averages. The overall average that will create the centre line of the X chart is the simple average of all of the measures. Calculate the moving range as the difference between each measure and the one before it.

2. Calculate the upper and lower control limits for the two charts. This first requires calculating the standard deviation, S, of the whole set of measures.
   Note that this chart, whilst the easiest to calculate and using fewer measures, also gives the least sensitive results and should only be used when limited measures are available.

Fig. 1. Calculating X/MR control limits

Next time: Calculation detail for X-bar/R Control Charts

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