Minitab trick / tip
This simple problem recently puzzled me one night, to the point where I couldn’t sleep. This made me a slave to coffee until I solved it:
How do I overlay two normal curves that have the same mean, but different standard deviations?
For example, I wanted one curve to use the within-subgroup standard deviation and the other curve to be based on the overall standard deviation, as in a Capability Analysis.
Solution
Suppose the mean is 500, the first standard deviation is 0.6, the second standard deviation is 0.57, and C1-C2 are empty.
1. Choose Graph>Probability Distribution Plot>Two Distributions.
2. Under Distribution 1, choose Normal. In Mean, enter 500. In Standard deviation, enter 0.6.
3. Under Distribution 2, choose Normal. In Mean, enter 500. In Standard deviation, enter 0.57. Click OK.
All is revealed in these screenshots:
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