The roles that are available for variables in the Reliability Growth launch window are determined by the specified data format tab. The roles are explained in this section.
Time to Event is the number of time units that elapse between the start of the test and the occurrence of an event (a failure or test termination). The test start time is assumed to be time zero. Note that the Time to Event role is available only on the Time to Event Format tab.
Two conventions are allowed (See Exact Failure Times versus Interval Censoring.):
• A single column can be entered. In this case, it is assumed that the column gives the exact elapsed times at which events occurred.
• Two columns can be entered, giving interval start and end times. If an interval’s start and end times differ, it is assumed that the corresponding events given in the Event Count column occurred at some unknown time within that interval. We say that the data are interval-censored. If the interval start and end times are identical, it is assumed that the corresponding events occurred at that specific time point, so that the times are exact (not censored).
The platform requires that the time columns be sorted in non-decreasing order. When two columns giving interval start and end times are provided, these intervals must not overlap (except at their endpoints). Intervals with zero event counts that fall strictly within a phase can be omitted, as they do not affect the likelihood function.
Timestamp is an absolute time (for example, a date). As with Time to Event, Timestamp allows times to be entered using either a single column or two columns. Note that the Timestamp role is available only on the Dates Format tab.
For times entered as Timestamp, the first row of the table is considered to give the test start time:
• When a single column is entered, the timestamp corresponding to the test start, with an event count of 0, should appear in the first row.
• When two columns, giving time intervals, are entered, the first entry in the first column should be the test start timestamp. See Phase.
Other details are analogous to those described for Time to Event Format in the section Time to Event. See also Exact Failure Times versus Interval Censoring.
Event Count is the number of events, usually failures addressed by corrective actions (fixes), occurring at the specified time or within the specified time interval. If no column is entered as Event Count, it is assumed that the Event Count for each row is one.
System ID identifies the prototype for an observation for multiple-prototype data. The System ID role is available only in the Concurrent Systems and Parallel Systems data formats. It is a required role for both data formats.
Reliability growth programs often involve several periods, or phases, of active testing. These testing phases can be specified in the optional Phase column. For the Time to Event Format and Dates Format data formats, the Phase variable can be of any data or modeling type. For the Parallel Systems data format, the data type of the Phase variable must be Numeric. For more information about structuring multi-phase data, see Test Phases. For an example, see Piecewise NHPP Weibull Model Fitting with Interval-Censored Data.
This produces a separate analysis for each value that appears in the column.