On variable sample size X̄ chart for processes with double assignable causes

Abstract

In statistical process control applications, variable sample size (VSS) X̄ chart is often used to detect the assignable cause quickly. However, it is usually assumed that only one assignable cause results in the out-of-control in the process. There are papers dealing with double assignable causes for traditional fixed sample size Shewhart chart. In this paper, we consider double assignable causes to occur with compound in the process and adopt the Markov chain approach to investigate the statistical properties of VSS X̄ chart. A procedure that can compute the optimal sample sizes is proposed.