What does the Cpk index measure in a process?

The Cpk index measures the process capability index, which reflects how centered a process is with respect to its specifications. It evaluates the process performance in relation to the specification limits and indicates how well the process is performing considering both its mean and the variation. A higher Cpk value signifies that the process is better centered within its specifications, thus leading to fewer defects and higher quality, which is essential in Six Sigma methodologies.

In contrast, the other options address different aspects of a process or project. For instance, measuring the average output relates to productivity but does not account for how closely that output meets specifications. The total cost during a project focuses on financial metrics rather than process capability. Lastly, counting the number of defects in a production run is relevant for quality assessment but does not provide information about the relationship between the process mean and its specification limits.