Chapter 9

Evaluation of Measurement Processes

Determining the capability of a measurement system is an important aspect of most process and quality improvement efforts.

(Burdick et al., 2003, p. 342)

9.1 Overview

The measure phase in a Six Sigma process improvement project is of crucial importance. It is natural therefore that teams working on projects need to be confident that the measurement processes they employ are sound and effective. Discussion of measurement processes has been postponed until this stage in the book so that use can be made of concepts from statistical models, of knowledge of designed experiments, and of components of variance in particular.

First the measurement of continuous variables will be considered. Following the introduction of the concepts of bias, linearity, repeatability and reproducibility, reference will be made to the problem of inadequate measurement units and how it manifests itself in control charts for variability. Gauge repeatability and reproducibility studies will be described and associated indices of measurement system performance introduced. Finally some reference will be made to scenarios involving attribute data.

9.2 Measurement Process Concepts

9.2.1 Bias, Linearity, Repeatability and Reproducibility

A measurement system may be defined as ‘the collection of instruments or gages, standards, operations, methods, fixtures, software, personnel, environment and assumptions used to quantify a unit of measure or fix assessment to the feature ...

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