ALTA Test Plan Types

The ALTA Test Plan utility allows you to choose a test plan type that will best fit your test goals and resource constraints. Which test plans are available will depend on whether you chose to design a one-stress or two-stress test in the Number of Simultaneous Stresses drop-down list. Each test plan will attempt to minimize the variance on the BX% life you specified in the BX% Life Estimate Sought field.

If you selected to design a single-stress test, the following test plans will be available:

2 Level Statistically Optimum Plan
Recommends two stress levels: a high stress level, which is the maximum allowable stress value that you specified during setup, and a low stress level. The low stress level and the proportion of units allocated to each stress level are calculated in order to minimize the variance on the BX% life.
3 Level Best Standard Plan
Recommends three stress levels: a high stress level, which is the maximum allowable stress value that you specified during setup, a low stress level and a mid stress level. The low and mid levels are calculated in order to minimize the variance on the BX% life. The test units are allocated equally to each stress level.
3 Level Best Compromise Plan
Recommends three stress levels using the same approach described above for the 3 Level Best Standard Plan. You define the proportion of test units to allocate to the mid stress level during setup in the Proportion of Units at Mid Stress field. The other proportions are calculated in order to minimize the variance on the BX% life.
3 Level Best Equal Expected Number Failing Plan
Recommends three stress levels using the same approach described above for the 3 Level Best Standard Plan. The proportion of units allocated to each stress level is calculated such that the number of units expected to fail at each stress level is equal.
3 Level 4:2:1 Allocation Plan
Recommends three stress levels using the same approach described above for the 3 Level Best Standard Plan. The proportion of test units tested at the high, mid and low stress levels will be calculated to be as close as 4:2:1 as possible (e.g., if you specified that 7 units are available in the Total Number of Units field, then this plan will recommend testing 4 units at the high level, 2 at the mid level and 1 at the low level).
- When you select this plan, you must also enter a value in the Low Stress Adjustment Factor field. The reduction factor must be a decimal between 0 and 1. The lower the value, the closer the recommended low stress level will be to the product's specified use stress level. Thus, if the reduction factor is 0, then the low stress level will be equal to the use stress level. If the reduction factor is 1, then the low stress level will equal the stress value that minimizes the variance on the BX% life.

If you selected to design a two-stress test, the following test plans are available:

3 Level Optimum Plan
Recommends three stress levels, each consisting of a combination of stress values for the two stress types. The combinations are structured such that:
- The 1st combination has both Stress 1 and Stress 2 at the maximum allowable stress values that you specified during setup.
- The 2nd combination will have Stress 1 at a calculated mid stress level and Stress 2 at the use stress level.
- The 3rd combination will have Stress 1 at the use stress level and Stress 2 at a calculated mid stress level.

The mid stress level is calculated such that the units tested at the 2nd and 3rd stress combinations will have the same probability of failure at the end of the test. The proportion of units allocated to each stress level is computed in order to minimize the variance on the BX% life.

5 Level Best Compromise Plan
Recommends five stress levels, each consisting of a combination of stress values for the two stress types. The combinations are structured such that:
- The 1st combination will have both Stress 1 and Stress 2 at the maximum stress values that you specified during setup.
- The 2nd combination will have Stress 1 at a calculated mid stress value A and Stress 2 at the use stress level.
- The 3rd combination will have Stress 1 at the use stress level and Stress 2 at a calculated mid stress value B.
- The 4th combination will have Stress 1 at the specified maximum stress value and Stress 2 at a calculated mid stress value C.
- The 5th combination will have Stress 1 at a calculated mid stress value D and Stress 2 at the specified maximum stress value.

The mid stress values A and B are calculated such that the units tested at the 2nd and 3rd stress combinations will have the same probability of failure at the end of the test. The mid stress values C and D are calculated such that the units tested at the 4th and 5th stress combinations will have the same probability of failure at the end of the test. The proportion of units allocated to each stress level is computed in order to minimize the variance on the BX% life.