Quality and Reliability Methods - SAS

338 Reliability Growth Chapter 18
Fit Model Options
Show MTBF Plot
The MTBF plot and an Estimates report open by default when the Piecewise Weibull NHPP option is
chosen (Figure 18.16). When Time to Event Format is used, the axes are logarithmically scaled. For further
details on the plot, see “Show MTBF Plot” on page 330.
Estimates
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” on page 347. The
likelihood function reflects the additional parameters and the constraints on the cumulative numbers of
events.
Show Intensity Plot
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” on page 331.
Show Cumulative Events 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” on page 332.
Show Profilers
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” on page 333.
It is important to note that, due to the default resolution of the profiler plot, discontinuities do not display
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 18.17.)
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 18.18, where
we have specified 500 as the Number of Plotted Points.)