The whole concept of calculating sigma score is flawed as we have established quite well on this Forum. The bottom line is that we are trying to establish the capability of a process. If the data is continuous, you can do process capability for any type of data, normal or not. If it is discrete you can also calculate a process capability. Minitab handles both situations well. Usually when you shift from continuous to discrete for sigma calculations you are going from some continuous value to a defective count. Even then there is some assumption of the normal approximation to the binomial (defective/not defective). In all cases, the calculations don’t provide much and when you add the dreaded 1.5 shift it is just an exercise.
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