Although the measurements given so far are perfectly adequate, most people use what are called capability indices which express the same thing, but in slightly different terms. The two most commonly used capability indices are Cp and Cpk, which respectively correspond to the measures of elbow room and distance to nearest specification limit which we have just discussed.

Cp is calculated by the formula:

This means that if the specified tolerance is 6 sigma units, Cp will be 1. A value of less than 1 means that the process will be incapable of meeting the specifications. For the reasons given earlier, most companies will demand a Cp value of at least 1.33, and sometimes 1.5.

As you might expect, Cpk is the minimum of the two values:

If the process is centred precisely on the target value, then Cpk will be the same as Cp. Generally, in practice, it will be slightly lower, and companies tend to require a Cpk of about 1.2.