Quality and Reliability Methods - SAS

Chapter 18 Reliability Growth 341
Fit Model Options
Show Intensity Plot
The Intensity plot shows the estimated intensity functions for the phases, along with confidence bands.
Since the intensity functions are computed based only on the data within a phase, they are discontinuous at
phase transitions. Color coding facilitates differentiation of phases. For further details, see “Show Intensity
Plot” on page 331.
Show Cumulative Events Plot
The Cumulative Events plot for the Reinitialized Weibull NHPP model portrays the estimated cumulative
number of events, with confidence bounds, over the design phases in the following way. Let t represent the
time since the first phase of testing began. The model for the phase in effect at time t is evaluated at time t.
In particular, the model for the phase in effect is not evaluated at the time since the beginning of the specific
phase; rather it is evaluated at the time since the beginning of the first phase of testing.
At phase transitions, the cumulative events functions will be discontinuous. The Cumulative Events plot
matches the estimated cumulative number of events at the beginning of one phase to the cumulative
number at the end of the previous phase. This matching allows the user to compare the observed cumulative
events to the estimated cumulative events functions. Color coding facilitates differentiation of phases.
Show Profilers
Three profilers are displayed, showing estimated MTBF, Failure Intensity, and Cumulative Events. Note
that the Cumulative Events Profiler is constructed as described in the Cumulative Events Plot section. In
particular, the cumulative number of events at the beginning of one phase is matched to the number at the
end of the previous phase. For further details, see “Show Profilers” on page 338
Piecewise Weibull NHPP Change Point Detection
The Piecewise Weibull NHPP Change Point Detection option attempts to find a time point where the
reliability model changes. This may be useful if you suspect that a change in reliability growth has occurred
over the testing period. Note that detection only seeks a single change point, corresponding to two potential
phases.
This option is available only when:
• a single column has been entered as Time to Event or Timestamp in the launch window (indicating that
failure times are exact), and
• a Phase has not been entered in the launch window
When the Piecewise Weibull NHPP Change Point Detection option is selected, the estimated model plot
and confidence bands are added to the Cumulative Events report under Observed Data. The Model List
updates, giving statistics that are conditioned on the estimated change point. Under Models, a Piecewise
Weibull NHPP Change Point Detection report is provided.
The default Piecewise Weibull NHPP Change Point Detection report shows the MTBF plot and Estimates.
(See Figure 18.20, which uses the data in BrakeReliability.jmp, found in the Reliability subfolder.) Note that
the Change Point, shown at the bottom of the Estimates report, is estimated as 12/21/2011. The standard