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

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 don't enter an upper specification

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.

- Drift scenario
- The drift scenario applies a multiplier to the standard deviations of all the elements, thus modeling dynamic shifts in the element means. These multipliers increase the gap standard deviation, thus moving the gap distribution outwards in both tails.
- Shift scenario
- The shift scenario assumes a static shift in all elements. In the shift scenario, shortages in the gap distribution are made up by adjusting the element means, rather than their standard deviations. Adjusting means is usually easier than adjusting standard deviations. Each element is shifted 1.5 standard deviations in the direction that moves the gap distribution closer to the lower specification.