Calculation detail for X-bar/R Control Charts
Calculate basic averages (X- bar) from measured values,
averaging the values in each subgroup and also measuring the range, R (the
maximum minus the minimum values in the subgroup). Then average these averages
to give the grand average (X-double bar) and the average range (R-bar). The
grand average and the average range give the central ‘average’ line in the
X-bar and the R charts.
Calculate upper and lower control
limits for each of the two charts. This requires the use of a ‘magic’
constants, A2, D3 and D4, which are simply
looked up in the table, based on the number of measures in each subgroup. The
control limits are calculated then using the formulae as in Fig. 2.
Calculating X-bar/R control limits
Next time: Calculation detail for
X-bar/S Control Charts
in Quality World, the journal of the Institute for Quality Assurance