The Use of Runs to Control the Mean in Quality Control

Abstract For quality control charts controlling the mean of a normal population, either small samples are taken out at frequent intervals or large samples at less frequent intervals. It will be shown that in order to detect small changes of the population mean, the amount of inspection is greatly reduced by the selection of large samples. However, if for other reasons small samples are desirable, a control by runs of sample means above or below certain control limits makes it possible to use small samples and yet maintain the advantage of a reduced amount of inspection. For certain types of runs, the sample size n = 1 turns out to be very economical, so that time saving methods of control by gauging may be introduced without appreciable loss of efficiency.