preface to fifteenth edition

2.138 SECTION 2
situation where relatively large numbers of units or small groups are to be compared, the control
chart is a simple means of indicating whether any unit or group is out of line. Thus laboratories,
production machines, test methods, or analysts may be put arbitrarily into a horizontal sequence.
Usually it is better to plot the means of small groups of observations on a control chart, rather
than individual observations. The random scatter of averages of pairs of observations is 1/(2) 1/2
0.71 as great as that of single observations, and the likelihood of two “wild” observations in the
same direction is vanishing small. The groups of two to five observations should be chosen in such
a way that only change variations operate within the group, whereas assignable causes are sought
for variations between groups. If duplicate analyses are performed each day, the pairs form logical
groups.
Some measure of dispersion of the subgroup data should also be plotted as a parallel control
chart. The most reliable measure of scatter is the standard deviation. For small groups, the range
becomes increasingly significant as a measure of scatter, and it is usually a simple matter to plot the
range as a vertical line and the mean as a point on this line for each group of observations.
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