Because of the nature of Cpk, we need to calculate two values (sometimes called CPU and CPL). These represent the process capability with respect to the upper and lower tolerance limits respectively. If the process mean is exactly halfway between the tolerance limits, then these two values will be the same (and they will also be the same as the Cp value). However, in practice, the process mean will usually be closer to one tolerance limit or the other. In this case, the final Cpk value will be the lower of the two values.
The next step, therefore, is to calculate the distance from each tolerance
limit to the process mean. The values for the upper tolerance limit (UTL),
lower tolerance limit (LTL), and the grand mean
(
)
which have already been calculated elsewhere, have been entered into the
appropriate boxes below. Complete the calculations, and enter your results
into the boxes outlined in red below. When you are ready, click the button
to check your answers.