Understanding Capability Measures
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There are two basic types of capability measures: those that measure long-term (LT) performance, and those that measure short-term (ST) performance. LT means that the process has had ample opportunity to exhibit typical shifts and drifts, cyclical patterns, and opportunities for special causes of variation. ST means that the process is devoid of those characteristics. Thus, ST is meant to measure the potential capability of the process if the process were performing at its absolute best. LT is meant to measure how the process actually performs over a long period of time in the presence of shifts, drifts, cyclical patterns, special causes, and so on.

Thus, one could say that a process has four possible states.

Condition A depicts the process performing at its optimum. Here, the process is on target and does not exhibit any type of dynamic drifting or other patterns. Hence, Condition A best portrays the potential capability of the process, because every process output has the potential to be on target and to be devoid of dynamic patterns and drift.

Condition B does not portray the absolute potential of the process because it is not on target.

Condition C is not usually an accurate portrayal of the actual performance of a process over a long period of time, because it assumes that the process is on target and does not account for shifts in the process mean.

Condition D better depicts the actual performance of the process over a long period of time. The process is not assumed to be on target and exhibits dynamic patterns and drift.

Therefore, when calculating ST capability statistics or indices, use condition A. When calculating LT capability statistics and indices, use condition D.