The way that Minitab calculates the gap pools depends on whether you enter an upper specification in the Calculate Gap Pools dialog box, and on whether you enter a column of drift factors.
When you enter an upper specification, both shift and drift scenarios are protected against. First, the mean pool is established to protect against static shifts. By default, each element is shifted 1.5 standard deviations in the direction that moves the gap distribution closer to the nearest specification limit.
After Minitab adjusts for the mean pool (and determines what the optimal mean pool is), the drift scenario is protected against. By default, each standard deviation is inflated by a factor of 1.8.
When you do not enter an upper specification, you can use either a drift scenario or a shift scenario.
To use a drift scenario, enter a column of drift factors (also called variation expansion factors). To use a shift scenario, don't enter a column of drift factors.