The Piecewise Weibull NHPP model can be fit when a Phase column specifying at least two values has been entered in the launch window. Crow-AMSAA models are fit to each of the phases under the constraint that the cumulative number of events at the start of a phase matches that number at the end of the preceding phase. For proper display of phase transition times, the first row for every Phase other than the first must give that phase’s start time. See Multiple Test Phases.
When the report is run, the Cumulative Events plot updates to show the piecewise model. Blue vertical dashed lines show the transition times for each of the phases. The Model List also updates. See Figure 9.17, where both a Crow-AMSAA model and a piecewise Weibull NHPP model have been fit to the data in TurbineEngineDesign1.jmp. Note that both models are compared in the Model List report.
Figure 9.17 Cumulative Events Plot and Model List Report
By default, the Piecewise Weibull NHPP report shows the estimated MTBF plot. The phases are differentiated with different colors. The Estimates and Covariance Matrix reports are shown below the plot. (See Figure 9.18.)
Figure 9.18 Piecewise Weibull NHPP Report
The MTBF plot and the Estimates report appear by default when the Piecewise Weibull NHPP option is chosen (Figure 9.18). When Time to Event Format is used, the axes are logarithmically scaled. For further details about the plot, see MTBF Plot.
The Estimates report gives estimates of the model parameters. Note that only the estimate for the value of λ corresponding to the first phase is given. In the piecewise model, the cumulative events at the end of one phase must match the number at the beginning of the subsequent phase. Because of these constraints, the estimate of λ for the first phase and the estimates of the βs determine the remaining λs.
The method used to calculate the estimates, their standard errors, and the confidence limits is similar to that used for the simple Crow-AMSAA model. For further details, see Parameter Estimates for Crow-AMSAA Models. The likelihood function reflects the additional parameters and the constraints on the cumulative numbers of events.
The Intensity plot shows the estimated intensity function and confidence bounds over the design phases. The intensity function is generally discontinuous at a phase transition. Color coding facilitates differentiation of phases. If Time to Event Format is used, the axes are logarithmically scaled. For further details, see Show Intensity Plot.
The Cumulative Events plot shows the estimated cumulative number of events, along with confidence bounds, over the design phases. The model requires that the cumulative events at the end of one phase match the number at the beginning of the subsequent phase. Color coding facilitates differentiation of phases. If Time to Event Format is used, the axes are logarithmically scaled. For further details, see Show Cumulative Events Plot.
Three profilers are displayed, showing estimated MTBF, Failure Intensity, and Cumulative Events. These profilers do not use logarithmic scaling. For more detail on interpreting and using these profilers, see the section Show Profilers.
It is important to note that, due to the default resolution of the profiler plot, discontinuities are not displayed clearly in the MTBF or Failure Intensity Profilers. In the neighborhood of a phase transition, the profiler trace shows a nearly vertical, but slightly sloped, line; this line represents a discontinuity. (See Figure 9.19.) Such a line at a phase transition should not be used for estimation. You can obtain a higher-resolution display making these lines appear more vertical as follows. Press Ctrl while clicking in the profiler plot, and then enter a larger value for Number of Plotted Points in the dialog window. (See Figure 9.20, where we have specified 500 as the Number of Plotted Points.)
Figure 9.19 Profilers
Figure 9.20 Factor Settings Window
