Cpk differs from Cp because the calculation for Cpk uses what?

The distinction between Cpk and Cp lies primarily in how they incorporate the process mean into their calculations. While Cp measures the potential capability of a process by using only the process standard deviation and the specification limits, Cpk takes into account how centered the process is within those limits.

Cpk is defined as the minimum of the two potential capability indices that consider how far the process mean is from the target value and how much variation exists in the process. By factoring in the process mean, Cpk assesses how well the process is performing relative to the specifications, providing a more accurate representation of a process's capability when the mean is not aligned perfectly within the specification limits.

This makes Cpk particularly useful for understanding not just the variation in the process but also the location of that variation in relation to the desired specifications, which is crucial for quality control and continuous improvement efforts.