Obtain Prior Estimates
To create an accelerated life test design, you must provide prior estimates of the parameters. Here is an approach to obtaining prior estimates:
1. Use your process knowledge to create a table of hypothetical, but likely, failure times at a small number of stress levels.
2. Use the Fit Life by X platform to fit a model and obtain estimates of the model parameters.
3. Use these estimates as your prior values for creating a design using the ALT Design platform.
Following the approach outlined above, you create a data table containing estimates of the number of failure cycles for a balanced design. Your table consists of five units at each of the three stress levels to be used in your design. Your data table is Torque Prior.jmp, found in the Sample Data Library.
1. Select Help > Sample Data Library and open Design Experiment/Torque Prior.jmp.
2. Select Analyze > Reliability and Survival > Fit Life by X.
3. Select Cycles and click Y, Time to Event.
4. Select Torque and click X.
5. From the Relationship list, select Log.
6. Enter 35 for the Use Condition.
7. From the Distribution list, select Weibull.
Figure 22.2 Fit Life by X Launch Window
8. Click OK.
9. Scroll down to the Weibull Results outline and open the Covariance Matrix outline.
Figure 22.3 Fit Life by X Model for Prior Data
The model for the mean is given in the Estimates outline. The Estimate column contains the parameter estimates for the intercept (β0), the linear coefficient (β1), and the scale (σ). The estimated variances and covariances for the parameter estimates are given in the Covariance Matrix outline. Use these parameter estimates and their covariances as your prior values when you construct your ALT design.