Quality and Reliability Methods - SAS

Chapter 18 Reliability Growth 325
Example Using the Reliability Growth Platform
Model-fitting in the Reliability Growth platform relies on the likelihood function. The likelihood function
takes into account whether interval-censoring is present or not. So, mixing interval-censored with exact
failure times is permitted.
Failure and Time Termination
Test Phases
A test plan can call for test termination once a specific number of failures has been achieved or once a
certain span of time has elapsed. For example, a test plan might terminate testing after 50 failures occur.
Another plan might terminate testing after a six-month period.
If testing terminates based on a specified number of failures, we say that the test is failure terminated. If
testing is terminated based on a specified time interval, we say that the test is time terminated. The
likelihood functions used in the Reliability Growth platform reflect whether the test phases are failure or
time terminated.
Reliability growth testing often involves several phases of testing. For example, the system being developed or
the testing program might experience substantial changes at specific time points. The data table conveys the
start time for each phase and whether each phase is failure or time terminated, as described below.
Single Test Phase
When there is a single test phase, the platform infers whether the test is failure or time terminated from the
time and event count entries in the last row of the data table.
• If the last row contains an exact failure time with a nonzero event count, the test is considered failure
terminated.
• If the last row contains an exact failure time with a zero event count, the test is considered time
terminated.
• If the last row contains an interval with nonzero width, the test is considered time terminated with
termination time equal to the right endpoint of that interval.
Note: To indicate that a test has been time terminated, be sure to include a last row in your data table
showing the test termination time. If you are entering a single column as Time to Event or Timestamp, the
last row should show a zero event count. If you are entering two columns as Time to Event or Timestamp,
the right endpoint of the last interval should be the test-termination time. In this case, if there were no
failures during the last interval, you should enter a zero event count.
Multiple Test Phases
When using Time to Event Format, the start time for any phase other than the first should be included in
the time column(s). When using Dates Format, the start times for all phases should be included in the time
column(s). If no events occurred at a phase start time, the corresponding entry in the Event Count column
should be zero. For times given in two columns, it might be necessary to reflect the phase start time using an
interval with identical endpoints and an event count of zero.