Assumptions of Process Capability
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There is no capability unless the process is in control. In order to make proper use of the capability statistics. You should verify that the process is stable by checking a control chart for the location (such as an Xbar chart or an I chart) and one for the variation (such as an R chart, an S chart or an MR chart). One advantage of Capability Sixpack is that it includes the appropriate charts as part of its output.

Validity of statistics is based on validity of assumed distribution. When observations come from a skewed distribution, the capability indices based on normal distribution such as Ppk and Cpk may look "good", yet many units will still be "out of spec." When the distribution of your data is not known, use Individual distribution identification to select the distribution that best fits your data. Minitab also allows you to transform your data to follow normal distribution using Box-Cox or Johnson transformation methods. See [2], [4], [5], [6], [9], [10], and [11] for a discussion.