Default Analysis
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When you do not enter an upper specification for the assembly gap, you have a choice between selecting a shift scenario or a drift scenario. The shift scenario assumes a static shift in all elements. The drift scenario applies a multiplier to the standard deviations of all of 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.

If you do not specify which scenario to use (entering a column of drift factors, or variation expansion factors, selects the drift scenario), the shift scenario is the default. This shift scenario is chosen so that shortages in the gap distribution can be made up by adjusting the element means, rather than their standard deviations. It is usually easier to adjust means than to adjust standard deviations. Each element is shifted 1.5 standard deviations in the direction that moves the gap distribution closer to the lower specification.

If an upper specification is given for the assembly gap, then 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 adjusting for the mean pool (and determining 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.