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Six Sigma Quality Improvement with Minitab, Second Edition by G. Robin Henderson

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Chapter 6

Process Capability Analysis

In general, process capability indices have been quite controversial.

(Ryan, 2000, p. 186)

Overview

Capability indices are widely used in assessing how well processes perform in relation to customer requirements. The most widely used indices will be defined and links with the concept of sigma quality level established. Minitab facilities for capability analysis of both measurement and attribute data will be introduced.

6.1 Process Capability

6.1.1 Process Capability Analysis with Measurement Data

Imagine that four processes produce bottles of the same type for a customer who specifies that weight should lie between 485 and 495 g, with a target of 490 g. Imagine, too, that all four processes are behaving in a stable and predictable manner as indicated by control charting of data from regular samples of bottles from the processes. Let us suppose that the distribution of weight is normal in all four cases, with the parameters in Table 6.1. The four distributions of weight are displayed in Figure 6.1, together with reference lines showing lower specification limit (LSL), upper specification limit (USL) and Target (T). How well are these processes performing in relation to the customer requirements?

Figure 6.1 Distributions of weight for the four processes.

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In the long term the fall-out, in terms of nonconforming bottles, would be as shown in the ...

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