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)/(3σ).  If the mean is closer to the lower specification limit (LSL),  Cpk is (X̄ – USL)/(3σ):

Acceptable minimum values for Cpk range from 1.33 to 2.0.