I am used JMP 9 and have noticed the "short term" sigma and Cpk result differ depending if the cability study is initiated from Anaylze -> Distributions or from Graph -> X-Bar.

I have data with fixed subgroup size of 6 units.

If I run a capability study from the distribution/histogram report I get a one short term sigma. The short term Cpk result does not match the result when the same data is run on minitab.

If I run a capaility study from the Xbar-R report I get a different short term sigma (control chart sigma). It is lower than the reported sigma from the distribution analsys. The short term Cpk results matches the result when the same data is run on minitab.

Why would I be getting different results for the same analysis? Does JMP estimate sigma differently depending on which report the capability study is initated from? Which results is correct?

Attached is the data set being used.

Thanks