# Stress levels for accelerated life test plans

You must enter the design and test stress levels. By default, Minitab determines an optimum allocation of units across stress levels.

## What is a stress level?

The stress level is the amount of stress that you put on a unit during a pass/fail test (probit analysis) or an accelerated life test. For example, you might use a number of stress values to test under common stress loads. To test a drill press that runs at 4 different speeds under typical use conditions, include the 4 different stress levels in your test.

When the expected life is too long to test practically, you can use stress levels higher than typical use conditions to cause early failures. For example, an engine has an average life of thousands of hours at a standard speed. To cause early failures, engineers test the engine at twice the standard speed. The engineers finish the test in a fraction of the time. Then, the engineers extrapolate time to failure at the normal use conditions from the elevated speed in the test.

Use your knowledge of the system to choose elevated stress levels. Stress levels that are too high may cause the system to suddenly fail instead of accelerating wear. For example, a stress test of a paper egg carton's compression strength at several thousand pounds of pressure would cause an immediate breakdown. This test would not offer much insight into the carton's strength under normal conditions.

## Allocation of test units across stress levels

You must enter the design and test stress levels. By default, Minitab determines an allocation of units across stress levels that minimizes the asymptotic variance of the parameter you choose to estimate. Alternatively, you can provide the allocation.

The efficiency of the test plan is measured in terms of the variance of the parameter that you want to estimate. However, a highly efficient test plan (one with small variance) can produce parameter estimates that are not accurate. In particular, accurate results depend on having enough failures at each stress level. To get accurate results, a common guideline is that the expected number of failures at each of the test stresses should be 4 or more.

The most efficient plan is only the most efficient in the specified search space. Minitab can determine the most efficient allocation of test units in two ways:
• You specify the search space as a finite set or sets of proportions. Each column represents a different test plan. Minitab ranks those test plans by their efficiency.
• Minitab searches for the optimum proportions in ranges. That is, Minitab uses a step to go from one candidate group of proportions to the others. The default step length is 0.05, but you can increase or reduce the length.

## Default ranges for proportionate allocation

Minitab chooses the pre-fixed ranges for the proportionate allocation of test units with these 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 range for the proportions at the lowest stress is RL = [0.05, 0.85]. The range for the proportions at the highest stress is RH = [0.075, 0.5].
• 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].
• Usually, if your design has K test stresses, the ranges for the proportions at the different stress levels are as follows:
• The range for the lowest test stress is RL = [1/K, 1/K + 0.35].
• The ranges for the middle stresses are R = [(1- 1/K - 0.350)/2K, 1/K].
• The range for the highest test stress is R = [(1- 1/K - 0.350)/2K, 1/K]. Minitab also chooses the proportion for the highest stress so that the complete set of proportions sums to one.

## Minitab finds the optimal allocation for each stress level

Minitab evaluates the efficiency of each plan and ranks them in order. By default, Minitab displays three different test plans. Use these steps to change the number of test plans Minitab displays.
1. Choose Stat > Reliability/Survival > Test Plans > Accelerated Life Testing.
2. Click Stresses.
3. Enter the number of best plans to output.
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