capability index (Cp)

Definition

Measure of the capability of a production process to produce parts within given upper and lower variability limits (tolerances). In a process that is in statistical control (has only common causes of variation), as the Cp increases so does the difference between what the process is capable of (see capability 2) producing and what it is required to produce. A Cp of 1 indicates 0.3 percent rejects or 3000 rejected parts per million (PPM), and is called the condition of Three Sigma. A Cp of 2 indicates 0.0007 percent rejects or 7 rejected ppm, a condition of six sigma. Also called process capability index, production capability index, or production capability ratio. Formula: (Upper variability limit – Lower variability limit) ÷ 6 x Standard deviation of the spread of variation.