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Statistical Advisor, How To Calculate Process Capability

Use PROCESS ANALYSIS. These statistical procedures include various indices of process capability, tolerance limits, and the standard process capability histogram.

The process capability indices are ratios that summarize the extent to which a manufacturing process or supplier turns out products or parts within specified engineering limits. For example, the so-called Cp index is the ratio of the specification range over the effective range manufactured by the process (typically defined by 6 times sigma, the standard deviation of the process).

Process Analysis software should allow you to summarize the process capability in the standard histogram form, showing the observed distribution of parts, the normal curve, and the specification limits. It should also compute tolerance limits, and estimate with a specified degree of certainty the percentage of manufactured parts that fall within the specification limits.