Lori, a software customer, phoned to ask if Cpk is the best statistic to use in a process that slits metal to exacting widths. As a PQ Systems technical support analyst, I too wondered what index would be best suited for her application, a highly specialized one. Perhaps Cpk, Ppk, Cpm, or some other index offers the best means of reporting the capability of her product or process.

Regardless of the index that is used, capability studies can evaluate how "good" the process is, with respect to target values or specifications. These studies should not be undertaken without sufficient understanding of the process. To determine which index to use, I asked Lori a few questions.

The most commonly used capability index is Cpk, which measures actual process capability for quality characteristics with one- or two-sided specification limits.¹

Lori pointed out that the process capability index, Cpk, has never dipped below 2 and typically averages above 3. Given this high degree of capability, she might consider reducing variation about the target.

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