Growth curves - exact data
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To use growth curves with exact data, you may enter the following information:

·    Time - the failure or retirement time of each sample.

·    System - identifies each system within a sample. You do not need a system column when you have only one system.

·    Retirement - indicates whether the data in each corresponding row is a failure time or a retirement time. Typically, the column will contain two distinct values; one representing failure times, one representing retirement times. By default, the lower value indicates the retirement time for a system. A retirement column is not necessary if all of your data are failure truncated or time truncated.

·    Frequency (or Cost) - the total cost of repairs or total frequency of failures at a particular time. You cannot use a cost column with Parametric Growth Curve. See Using cost or frequency columns for more information.

This data set illustrates exact data.

Repairable systems data can consist of:

·    Failure truncated data: Enter a column of failure times. The retirement time is the largest failure time. A failure-truncated system is retired once a certain number of failures occur. In a failure-truncated system, the system is retired immediately upon the last failure.

If the systems in the example above are failure-truncated, system 1 was retired following the failure at 9 hours.

·    Time truncated data: Enter a column of failure times. The retirement time for a system is the largest value in the variables (or time) column for that system. A time-truncated system is retired after a specified period of time. In a time-truncated system, the largest time is not a failure time.

If the systems in the example above are time-truncated and no retirement column is given, system 1 was retired at 9 hours.

·    Data with a retirement column: Enter a column of retirement indicators. Retirement indicators can be numeric, text, or date/time. If you do not specify which value indicates the retirement value in the Retirement subdialog box, Minitab assumes the lower of the two values indicates the retirement value, while the higher one indicates a failure/repair time. If specified, the retirement value applies to every sample in each analysis.

The columns for each sample must be the same length, although pairs of columns from different samples can have different lengths. Use retirement columns if one group is time-truncated and another is failure-truncated.

In the example above, system 1 failed at the first and fifth hour and is retired at the ninth hour.

See Using time and retirement columns for more information about the relationship between your time column and retirement times.

When you have more than one sample, you can use separate columns for each sample. Alternatively, you can stack all the samples in one column, then set up a column of grouping indicators. For an illustration, see Stacked vs. Unstacked data.

Each sample is analyzed independently and results in one growth curve. All the samples display on a single plot, with different colors and symbols to help you compare reliability growth between samples.

For general information on repairable systems data, see Data - Growth Curves.