C and U Subcommands (SPCHART command)

C produces a c chart and U produces a u chart. Both charts are control charts for attributes. That is, the charts use data that can be counted.

A c chart plots the total number of defects or nonconformities in each subgroup. A defect or nonconformity is one specification that an item fails to satisfy. Each nonconforming item has at least one defect, but any nonconforming item may have more than one defect. The center line of the c chart indicates the mean of the defect numbers of all subgroups. The control limits are based on the Poisson distribution and can be controlled by the user-specified SIGMAS value. When the sample sizes are not equal, c charts are not recommended.

A u chart plots the average number of defects or nonconformities per inspection unit within a subgroup. Each subgroup contains more than one inspection unit. The center line of the u chart indicates the average number of defects per unit of all subgroups. The control limits are based on Poisson distribution and can be controlled by the user-specified SIGMAS value.