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Statistical Advisor, Process Capability Analysis

Use PROCESS ANALYSIS. This chapter discusses when to plot the standard process capability histogram, to compute various capability indices, and to compute normal and distribution-free tolerance limits.

The general purpose of process capability analysis is to estimate the percentage of items (products, parts) that fall outside the specification limits. For example, one may compute the proportion of vegetables supplied to a restaurant that fall outside specification limits, that is, that are of such poor quality that they cannot be used in the kitchen.

A common summary of process capability is the histogram, with the normal curve and the specification limits superimposed. This histogram can serve as a quick check regarding the proportion of items that fall within specification limits. This information can also be expressed in various process capability ratios.

Facilities for computing tolerance limits should allow you to, for example, estimate the proportion of items that will fall within certain limits with a particular degree of certainty. Put another way, how sure are we that a certain proportion of items falls within a particular set of limits.