Economical Quality Control Policies for a Single Cause System

We consider four types of process control procedures; variables control charts, attributes control charts, a never-inspect policy, and a regular inspection policy consisting of periodic process reset and/or repair. Using a designed experiment, we compare the optimal economic designs associated with these policies for a practical range of costs and other system parameters. We find that the regular inspection policy is economically optimal in many configurations. Moreover, we find that the type of policy that yields minimum cost depends primarily on the expected magnitude of the process shift, the relative cost of variable versus attribute sampling, and the expected search time for an assignable cause. Some general guidelines for the appropriate type of control procedure for use in practice are given.