Statistical Methods for Quality Improvement, Third Edition

CHAPTER 8

Alternatives to Shewhart Charts

The Shewhart and Shewhart-type charts that were presented in Chapters 4–6 are the most commonly used control charts. Some have even argued that Shewhart charts are superior to other types of control charts/procedures and should be used exclusively. In particular, Deming (1993, p. 180) stated: “The Shewhart charts do a good job under a wide range of conditions. No one has yet wrought improvement.”

Times change, however, and charts with superior properties have been developed. This is to be expected; how many people would want to drive a car that was made in 1924, the year that Shewhart sketched out the idea for a control chart? These superior charts are discussed in considerable detail in this chapter.

See Woodall and Montgomery (1999) and Montgomery and Woodall (1997) for the same general view regarding the need to consider and use alternative control chart procedures. In particular, the latter stated: “In many cases the processes to which SPC is now applied differ drastically from those which motivated Shewhart's methods.”

8.1 INTRODUCTION

We would certainly want whatever procedure we use for controlling the process mean to enable us to detect a shift in the mean that is of any consequence as quickly as possible, but not produce a high rate of false signals. For example, we want to quickly detect a shift that would cause nonconforming units to be produced, but not to frequently receive signals suggesting a mean shift when in fact there ...