Process capability (PCIs) are used in industry to assess percentages o
f nonconforming parts. An underlying assumption is that the output pro
cess measurements are distributed as normal random variables. When nor
mal distributions are assumed, but different distributions are present
- such as skew, heavy-tailed, and short-tailed distributions - the pe
rcentages of nonconforming parts are significantly different than the
computed PCIs indicate. Data arising from nonnormal distributions can
sometimes be transformed to conform to the normality assumptions and t
he PCI's computed for the transformed data. In this paper, the effect
of the transformation of the estimate of nonconforming parts is examin
ed for three examples of nonnormal distributions - gamma, lognormal, a
nd Weibull. The results of this experimental analysis suggest that dat
a transformation can be useful for estimation an interval for C-pk val
ues and the number of nonconforming parts.