A note on the average run length of cumulative sum control charts for count data

This note presents preliminary results on the average run length (ARL) of a Cumulative Sum Control Chart (CUSUM) used to monitor defects or counts, when the count distribution is Neyman Type-A. The ARL for this distribution is contrasted to the more customary Poisson model for counts or defects. We show that the ARL curve of a Neyman Type-A CUSUM always lies below the ARL curve of a Poisson CUSUM.