Attribute charts for defects

Abstract

A continuing theme of SPC is that of controlling the process at a very early stage so that minor difficulties do not grow over time into major obstacles. Thus nonconforming units may be unacceptable because of the high level of minor nonconformities they contain. A small particle of dust included at the paint-spraying stage of a high-cost luxury limousine will appear as a real blemish to a customer who is expecting a mirror finish. Similarly aircraft navigation equipment will fail if a greasy thumb mark is registered in a critical item of the component at the assembly stage. Controlling and reducing the level of nonconformities is therefore just as important as monitoring the rejection rate for non-conforming units. The two charts used are the u chart and the c chart.