Requirements for Accelerated Life Testing

The following sections describe the minimum data requirements for using different folios in Accelerated Life Testing.

Life-Stress Data Folios

The minimum data requirements for life-stress data folios are broken down by model in the table below. Note the following:

• "Number of failures" assumes unique (i.e., different) failure times.

• "Number of stress levels" assumes unique stress levels.

• "Stresses" refers to the number of stress columns used in the analysis (this may not necessarily equal the number of stress columns in the data sheet).

• If the data set uses the minimum number of stress levels, then each stress level must have at least one failure.

• If you provide the value of the shape parameter (β for the Weibull distribution or σ for the lognormal distribution), less data will be required for calculation, as shown in the Give β or σ and Solve column of the minimum data requirements table below. If the data set you enter meets only these minimum requirements, when you click Calculate, you will be prompted to provide the value of the shape parameter in a window. Note that confidence bounds calculations are not possible in these situations.

Life-Stress Model	Give β or σ and Solve	Exponential	Weibull/Lognormal
Arrhenius / Eyring / Inverse Power Law	Failures = 2 Stress Levels = 2 Note: If you provide 3 or more failures at only 1 stress level, you can provide the acceleration factor to calculate.	Failures ≥ 2 Stress Levels ≥ 2	Failures ≥ 3 Stress Levels ≥ 2.
Temperature-Humidity / Temperature-Nonthermal	Failures = 3 Stress Levels = 3	Failures ≥ 3 Stress Levels ≥ 3	Failures ≥ 4 Stress Levels ≥ 3
Generalized Eyring	Failures = 4 Stress Levels = 4	Failures ≥ 4 Stress Levels ≥ 4	Failures ≥ 5 Stress Levels ≥ 4
Proportional Hazards / General Log-Linear	Failures = stresses + 1 Stress Levels = stresses + 1	Failures ≥ stresses + 1 Stress Levels ≥ stresses + 1	Failures ≥ stresses + 2 Stress Levels ≥ stresses + 1 Note: The lognormal distribution is not available for proportional hazards.
Cumulative Damage	Failures = stresses + 1 Stress Levels = stresses	Failures ≥ stresses + 1 Stress Levels ≥ stresses	Failures ≥ stresses + 2 Stress Levels ≥ stresses Note: Only one stress column can be used with the lognormal distribution.

If you enter no data at all and click the Calculate icon, Weibull++ will prompt you to enter the values of the parameters for the selected model, as shown next. The appearance of the Parameter Input window will vary depending on the life-stress model and distribution selected.

Specify values for the parameters based on the selected life-stress model and distribution and click OK. All options and calculations supported by Accelerated Life Testing will be available except calculations involving confidence bounds and plots that require individual stress levels.

Degradation Analysis Folios

The minimum data requirements for Accelerated Life Testing degradation analysis folios are as follows:

• The Unit ID must be entered for all data points.

• For all models except Gompertz, there must be at least two data points for each unique Unit ID. For the Gompertz model, there must be at least three data points for each unique Unit ID.

• Within each unique Unit ID, the inspection times must be increasing.

• The data must produce enough extrapolated failure/suspension times to perform accelerated life testing data analysis with the selected Accelerated Life Testing model. See the requirements for life-stress data folios.