Cpk is the most common measure of process capability for continuous data. Cpk is very useful because it takes *variation* (*σ*) and *position* (X̄) into account versus upper and lower specification limits. If the mean is closer to the upper specification limit (USL), then Cpk is (USL-X̄)/(3*σ*). If the mean is closer to the lower specification limit (LSL), Cpk is (X̄ – USL)/(3*σ*):

Acceptable* minimum* values for Cpk range from 1.33 to 2.0.