Attributes Control Charts with Large Sample Sizes

Journal of Quality Technology vol. 28 issue 4 - October 1996

Abstract:When a control chart for attributes data is based on a large sample size, one often finds that the control limits are narrow, and that many observations fall outside the control limits. The narrow control limits, which are caused by the large sample size, make evident the natural variation in the probability of producing a nonconforming unit from the underlying process. This paper discusses the different types of variation that affect these control charts for attributes data and the relative merits of plotting the proportion nonconforming as attributes (P chart) versus variables (individuals chart), and it develops objective criteria for making this choice. Finally, several examples are presented, using data from a business telephone service maintenance process.

Keywords: Attributes control charts - Statistical quality control (SQC) - Statistical process control (SPC) - Sampling