Statistical Methods for Quality Improvement, Third Edition

CHAPTER 4

Control Charts for Measurements With Subgrouping (for One Variable)

In this chapter we discuss control charts that can be used when subgroups (samples) of data are formed. We consider control charts that can be used when measurements are made and the values are obtained with sufficient speed to allow subgroups to be formed. Typical measurements are length, width, diameter, tensile strength, and Rockwell hardness.

Subgrouping is an important topic in itself, so subgrouping considerations and basic control chart principles are discussed first.

4.1 BASIC CONTROL CHART PRINCIPLES

Control charts can be used to determine if a process (e.g., a manufacturing process) has been in a state of statistical control by examining past data. This is frequently referred to as retrospective data analysis. We shall also refer to this stage as Stage 1. More importantly, recent data can be used to determine control limits that would apply to future data obtained from a process, the objective being to determine if the process is being maintained in a state of statistical control. This is termed Stage 2. (Note: Various writers have referred to these two stages as Phase 1 and Phase 2, respectively, and we will also use this terminology at times, such as when the terminology appears in the title of a cited paper.)

Control charts alone cannot produce statistical control; that is the job of the people who are responsible for the process. Control charts can indicate whether or not statistical control ...