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Quality and Reliability Methods - SAS

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Chapter 20 <strong>Reliability</strong> <strong>and</strong> Survival Analysis II 397<br />

Nonlinear Parametric Survival Models<br />

Figure 20.21 Initial Parameter Values in the Nonlinear Fit Control Panel<br />

6. Click Save Estimates.<br />

The parameter estimates in the column formulas are set to those estimated by this initial nonlinear<br />

fitting process.<br />

The Weibull column has the Weibull formula previously shown. To continue with the fitting process:<br />

7. Select Analyze >Modeling > Nonlinear again.<br />

8. Select Model <strong>and</strong> click X, Predictor Formula.<br />

9. Select Weibull loss <strong>and</strong> click Loss.<br />

10. Click OK.<br />

The Nonlinear Fit Control Panel on the left in Figure 20.22 appears. There is now the additional<br />

parameter called sigma in the loss function. Because it is in the denominator of a fraction, a starting<br />

value of 1 is reasonable for sigma. When using any loss function other than the default, the Loss is Neg<br />

LogLikelihood box on the Control Panel is checked by default.<br />

11. Click Go.<br />

The fitting process converges as shown on the right in Figure 20.22.

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