This month we are covering the calculations for average and range control charts - namely, X-bar/R charts. First, 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. Second, calculate upper and lower control limits for each of the two charts.
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