New Demerit Control Limits for Poisson Distributed Defects

Abstract

More than one type of defect may be found in complex products, and these defects are not always of equal severity. They can be categorized according to the degree to which they affect product quality, commonly measured as a weighted sum of each category's defects and considered an observation of demerits. The number of demerits is monitored using a demerit control chart. Traditionally, this chart is constructed with parameters based on the normal distribution, and 3-sigma limits are established. These control limits have been improved upon by other authors' use of the characteristic function of demerits to determine the actual probability distribution of the statistic and establish probability-based limits. In the present work, we present an alternative method for finding the exact distribution of demerits which gives results different from those previously produced with the characteristic function.

Keywords:

• Demerit control charts
• Ordinal variables
• Poisson distribution
• Statistical process control