Improved Self-Starting Control Charts for Short Runs

The traditional control charts are designed to minimize the out-of-control average run length (ARL), given a desired in-control ARL. When the out-of-control ARL is larger than the desired in-control ARL, the control chart is called biased. This paper investigates the bias of Shewhart Q charts proposed by Quesenberry [9] for detecting process mean shifts in start-up processes and short runs. Two improved schemes are proposed to alleviate the bias problem of the Shewhart Q charts.