Pre-fixed Ranges and Default Steps
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Minitab chooses the pre-fixed ranges for the proportionate allocation of test units based on the following criteria:

·    More test units are assigned to the lowest test stress.

·    Either a large or small number of test units exists at the middle stresses.

·    The proportionate allocation of units at a test stress is not too small relative to the others.

The ranges change as the number of stresses change:

·    For a two-stress design, the ranges for the proportions at the lowest and highest test stress are RL = [0.05, 0.85] and RH = [0.075, 0.5], respectively.

·    For a three-stress design, the lowest test stress range is RL = [0.333, 0.683]. The other test stresses have a common range of R = [0.040, 0.333].

·    In general, if your design has K test stresses, the range for the proportions at the

-    lowest test stress is RL = [1/K, 1/K + 0.35]

-    middle stresses is R = [(1- 1/K - 0.350)/2K, 1/K]

-    highest test stress is R = [(1- 1/K - 0.350)/2K, 1/K] and chosen so that the complete set of proportionate allocations sums to one