Statistical quality control often relies on the completion of a Phase I study. Many Phase I control charts, however, rely on an assumption of normally distributed process observations that may not be reasonable in the early stages of process control. A literature review shows no purely distribution-free Phase I control chart methods currently exist. This study introduces a distribution-free method for defining the in-control state of a process and identifying an in-control reference sample that can be used to estimate the process parameters for a Phase II procedure. The proposed procedure compares favorably with the traditional X-bar chart when the process is normally distributed and better than the X-bar chart when the process distribution is skewed or heavily tailed.
Available at: http://works.bepress.com/charles_champ/6/