Quality and Reliability Methods - SAS

328 Reliability Growth Chapter 18
Reliability Growth Platform Options
Mean Time Between Failures Plot Options
Clicking on the Mean Time Between Failures red triangle and then clicking on Options opens a window
that enables you to specify intervals over which to average.
Two types of averaging are offered:
• Equal Interval Average MTBF (Mean Time Between Failures) enables you to specify a common interval
size.
• Customized Average MTBF enables you to specify cut-off points for time intervals.
– Double-click within a table cell to change its value.
– Right-click in the table to open a menu that enables you to add and remove rows.
Reliability Growth Platform Options
Fit Model
Model List
The Reliability Growth red triangle menu has two options: Fit Model and Script.
This option fits various non-homogeneous Poisson Process (NHPP) models, described in detail below.
Depending on the choices made in the launch window, the possible options are:
• Crow AMSAA
• Fixed Parameter Crow AMSAA
• Piecewise Weibull NHPP
• Reinitialized Weibull NHPP
• Piecewise Weibull NHPP Change Point Detection
Once a model is fit, a Model List report appears. This report provides various statistical measures that
describe the fit of the model. As additional models are fit, they are added to the Model List, which provides
a convenient summary for model comparison. The models are sorted in ascending order based on AICc.
The statistics provided in the Model List report consist of:
Nparm
The number of parameters in the model.
-2Loglikelihood The likelihood function is a measure of how probable the observed data are, given the
estimated model parameters. In a general sense, the higher the likelihood, the better the model fit. It
follows that smaller values of -2Loglikelihood indicate better model fits.