I have a set of seal strength data (30 bags, 4 seals per bag as a subgroup for each bag) within a specification range of 0.5 to 2.5 and actual values of 1.3 to 1.7. The probability plot shows the data are not normally distributed. The Johnson transformation in Minitab transforms the data to acheive a p-value greater than 0.05 which is acceptable to me. When I perform a Six-Pack analysis of the data, I get a capability histogram (LSL: -3.61, USL:2.83) with normal probability plot (AD:0.487, P:0.208). I am concerned about the negative numbers on the capability histogram after applying the Johnson Transformation. Should I be concerned? I am wondering whether the best approach is to find a non-normal distribution that best fits the data. When I consider Weibull and Largest Extreme Value Distributions, neither is able to meet the 0.05 significance without removing the extreme outliers in the data set. I am not certain how to best approach analyses of these data and I am looking for help.
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