In the Reliability Growth platform, the Time to Event Format and the Dates Format enable you to enter either a single column or two columns as Time to Event or Timestamp, respectively. The Concurrent Systems and the Parallel Systems enable you to enter one column corresponding to each level of the System ID variable. There are examples of multiple-prototype data tables in the Reliability folder of the Sample Data folder: Concurrent Systems.jmp for Concurrent Systems and four tables with the prefix Parallel Systems for Parallel Systems.
This section describes how to use these two approaches to specify the testing structure.
In some testing situations, the system being tested is checked periodically for failures. In this case, failures are known to have occurred within time intervals, but the precise time of a failure is not known. The failure times are said to be interval-censored.
The Reliability Growth platform accommodates both exact, uncensored failure-time data, and interval-censored data. When a single column is entered as Time to Event or Timestamp, the times are considered exact times (not censored).
When two columns are entered, the platform views these as defining the start and end points of time intervals. If an interval’s start and end times differ, then the times for failures occurring within that interval are considered to be interval-censored. If the end points are identical, then the times for the corresponding failures are assumed to be exact and equal to that common time value. So, you can represent both exact and interval-censored failure times by using two time columns.
In particular, exact failures times can be represented in one of two ways: As times given by a single time column, or as intervals with identical endpoints, given by two time columns.
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.
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, the test is said to be failure terminated. If testing is terminated based on a specified time interval, the test is said to be 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.
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.
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.
In a multi-phase testing situation, the platform infers whether each phase, other than the last, is failure or time terminated from the entries in the last row preceding a phase change. Suppose that Phase A ends and that Phase B begins at time tB. In this case, the first row corresponding to Phase B contains an entry for time tB.
• If the failure time for the last failure in Phase A is exact and if that time differs from tB, then Phase A is considered to be time terminated. The termination time is equal to tB.
• If the failure time for the last failure in Phase A is exact and is equal to tB, then Phase A is considered to be failure terminated.
• If the last failure in Phase A is interval-censored, then Phase A is considered to be time terminated with termination time equal to tB.
The platform infers whether the final phase is failure or time terminated from the entry 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, or an interval with nonzero width, the test is considered time terminated. In the case of an interval, the termination time is taken as the right endpoint.