I am trying to assess capability on a process that can have adjustment done (hardware mean shift) so the critical parameter is how wide is the spread of data. Each machine produces 1 subgroup of 50 data points with summary mean and stdev statistics. Each machine must pass bilateral +/- limits on the variation=3 sigma and the mean is for reference only.

Before embarking on improvement activities, I need to have a proper and correct understanding of the current capability of this process and I need your advice on what would be a proper way to do this (process is in control and data is normal). What is important to me is to make sure the spread does not exceed the limits as I can always bring any mean shift back through adjustment.

1. I think calculating Cpk by using each subgroup mean value as 1 data point is wrong. By doing so, we lose the within subgroup variation component.

2. Calculating Cpk from variation= 3 sigma with limits 0 and USL is wrong as well as this is calculating variation of the variation, if you get what I mean.

3. The nearest logical measurement that I can think of to only report the Cp instead so we know the spread relative to the tolerance. By using the between/within formulas I can come up with summary Cp values that makes use of each subgroups' mean and stdev, without losing valuable data.

Pardon the long writeup above, I hope my description of the problem provides enough details for discussion.

Please comment if what I am doing is correct, or if you think there is a better or more accurate way to assessing a spec that controls variation. Thanks in advance!