Capability Analysis

Capability Analysis is the act of determining process capability, or the ability of a process to meet a designated set of specifications.

It involves a set of statistical calculations to determine whether the spread of the performance of a process fits within the desired specification.

There are several terms embedded within capability analysis.

Process Capability Ratio (C_p): This is a comparison of the size of the tolerance of the specification with the statistical spread of the process (σ).

Capability Index (C_pk): The process capability ratio is only a measure of relative size. It doesn’t tell whether the process spread is located properly. Best case, the spread is centered. In reality, more often than not, the center of the process spread does not align with the center of the specification limits. The capability index defines the location relationship.

In addition, you may also see Process Performance (P_p) and Process Performance Index (P_pk). In general, the biggest difference is that process capability is more short term, and process performance is more long term.