
232 A First Course in Quality Engineering
measure of variability� The ease in nding the square root of four was an
advantage in those days when electronic calculators were not available� He
therefore recommended a sample size of four�
Later users saw some advantage in using a sample size of ve, because it
provided some advantage in calculating the average� (Obviously, they did
not have electronic calculators, either�) Therefore, a sample size of four or ve
became the norm for X- and R-charts� In some situations, however, the choice
of sample size may be dictated by practical considerations, and the analyst
is forced to choose other sizes� ...