In my July column, “Where Do Manufacturing Specifications Come From?” we found that the intraclass correlation coefficient is the natural measure of relative utility. This measure is theoretically sound and easy to explain. This column will look at how to use the intraclass correlation to characterize the relative utility of a measurement system for use with a particular product.

The issue of relative utility has two components. The first of these is whether you can use a measurement system to characterize items or batches as conforming or nonconforming. The answer to this question was provided in my June column, “Is the Part in Spec?” and the answer was expanded in my July column mentioned above. In this column I will address the second aspect of relative utility, which is whether you can use your measurement system to track your process on a process behavior chart. This aspect of relative utility will consider such questions as: Can you detect process shifts when they occur? Can you track process improvements? When do you need to consider finding an alternative measurement system?

However, before we get to these questions, we need to have some notation in the interest of clarity.

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