P and NP Subcommands (SPCHART command)

P produces a p chart and NP produces an np chart. Both charts are control charts for attributes. That is, these charts use data that can be counted, such as the number of nonconformities and the percentage of defects.

A p chart plots the fraction nonconforming on a control chart. Fraction nonconforming is the proportion of nonconforming or defective items in a subgroup to the total number of items in that subgroup. This measurement is expressed as a decimal or, occasionally, as a percentage. The center line of the control chart is the mean of the subgroup fractions, and the control limits are based on a binomial distribution and can be controlled by the user-specified SIGMAS value.

An np chart plots the number nonconforming rather than the fraction nonconforming. The center line is the mean of the numbers of nonconforming or defective items. The control limits are based on the binomial distribution and can be controlled by the user-specified SIGMAS value. When the subgroup sizes are unequal, np charts are not recommended.