Specify the stress patterns used in the experiment using the second panel.
The Step Stress pattern has stress levels that are changed at arbitrary time points. The duration of each stress step and associated stress level must be specified in ascending time order.
The Stress Pattern panel in the Step Stress tab contains the following options:
Stress Duration
The column that contains the length in time units of each stress step.
Stress
The column that contains the level of the stress setting.
Pattern ID
The column that contains a unique identifier for the stress pattern. This column is used to match stress patterns in the Stress Pattern data table with the observations in the Time-to-Event data table.
Pattern Continuation
Specifies how to handle failures that occur after the final time period in the defined stress pattern. This panel contains the following options:
Terminate
A failure that occurs at a time beyond the final time period in the defined stress pattern produces an error, and the model is not fit.
Extend
A failure that occurs at a time beyond the final time period in the defined stress pattern assumes the same stress level as the level in the final time period.
Repeat
A failure that occurs at a time beyond the final time period in the defined stress pattern assumes the same stress level as if the stress pattern were being repeated. For example, if a failure occurs 10 time units after the final time period in the defined stress pattern, then the stress level at that failure time is set to the stress level at 10 time units after the beginning of the defined stress pattern.
Note: The default Pattern Continuation setting for the Step Stress Pattern is Terminate.
The Ramp Stress pattern defines stress as a linear function of time. Each pattern is defined by an intercept (the stress level at time zero) and a slope (the increase in the stress level for every one time unit). Each pattern is described in a single row in the stress pattern data table.
Intercept
The column that contains the intercept for each pattern.
Slope
The column that contains the slope for each pattern.
Pattern ID
The column that contains a unique identifier for the stress pattern. This column is used to match stress patterns in the Stress Pattern data table with the observations in the Time-to-Event data table.
The Sinusoid Stress pattern defines stress as a periodic function. The pattern is defined by a level, an amplitude, a period, and a phase. Each pattern is described in a single row in the stress pattern data table. The definition of the pattern is as follows:
S(t) = level + amplitude * sin(phase + (2*π*t) / period)
Level
The column that contains the level for each pattern.
Amplitude
The column that contains the amplitude for each pattern.
Period
The column that contains the period for each pattern.
Phase
The column that contains the phase for each pattern.
Pattern ID
The column that contains a unique identifier for the stress pattern. This column is used to match stress patterns in the Stress Pattern data table with the observations in the Time-to-Event data table.
The Piecewise Ramp Stress pattern defines stress as a piecewise linear function of time. The line segments for the stress level over time can be disjoint or continuous. Line segments can also be flat, so that step stress and ramp stress can be combined. The line segments are defined in the stress pattern data table by the time duration of the segment and the start and end levels of the stress setting.
Stress Duration
The column that contains the length in time units of each stress step.
Stress Ramp
The two columns that contain the stress levels at the start and end of the step.
Pattern ID
The column that contains a unique identifier for the stress pattern. This column is used to match stress patterns in the Stress Pattern data table with the observations in the Time-to-Event data table.
Pattern Continuation
The Pattern Continuation panel enables you to specify the stress levels that occur after the final time period in the defined stress pattern. This panel contains the following options:
Terminate
A failure that occurs at a time beyond the final time period in the defined stress pattern produces an error, and the model is not fit.
Extend
A failure that occurs at a time beyond the final time period in the defined stress pattern assumes the same stress level as the level in the final time period.
Repeat
A failure that occurs at a time beyond the final time period in the defined stress pattern assumes the same stress level as if the stress pattern were being repeated. For example, if a failure occurs 10 time units after the final time period in the defined stress pattern, then the stress level at that failure time is set to the stress level at 10 time units after the beginning of the defined stress pattern.
Note: The default Pattern Continuation setting for the Piecewise Ramp Stress Pattern is Terminate.