A Charting Technique for Control of Low‐Defective Production

The technology of modern‐day production processes has improved to such an extent that situations could arise in which the proportion of defective items is so low that the conventional p chart is no longer appropriate for process control. An alternative charting technique, based on more exact probability calculations, has been found to be suitable for controlling processes producing less than one per cent defective. Although the presence of three defectives per sample is tolerated so long as it does not occur more than twice in three consecutive samples, an alarm will immediately be sounded by one or two defectives occuring frequently. Thus the low p chart is more reliable, as there is no loss of reaction to out‐of‐control situations, yet less over‐reaction to very short runs of defectives.