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So I've been doing validation work for a while now so I'm resionabilly fimilar with CPK and other process indicators. However our ongoing validation has lead me into a problem.

I'm calculating a Cpk on a heat sealing process based on seal peeling strength evaluations. There is a lower limit of 1.5N/15mm strip with no upper limit.

When I look at the results there is a mean on 3.4N with a SD of 0.78, so not good; cpk of only 0.8:

But then I look at a distribution plot of the results and see that the distribution has a large tail on the positive side of the mean.

This tail is giving me a large SD which is causing a low CPK.

This dose not seem reasonable to me; the outliers/tail above the mean is making me fail the lower specification CPK.

What can I do in this situation?

If I calculate the SD only on the lower side of the mean i.e. (summation of: [3.4-{result less than 3.4}] /{n of results less than 3.4})^0.5.

I then get a SD of 0.37 and a CPK of 1.7

Is this reasonable to do? What else could I do?

Any help would be appreciated.