Life Data
Classifications
Life data of a component, product or system can be separated into
the following categories: complete data, right censored data, interval censored data, and
left censored data. Each data type requires a modification to the analysis in order to
correctly estimate the underlying lifetime distribution. A good life data analysis
package, like ReliaSoft's Weibull++, must be able to correctly handle all of these types
of data. The characteristics of each data type are presented next.
Complete Data
Complete data, as shown in Figure 1, indicates that all of the units under the
test failed and the time-to-failure for each unit is known. Therefore, complete
information is known regarding the entire sample.
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Figure 1: Complete |
Right Censored Data
Right censored data, also called suspended data, is composed of
units that did not fail during the test as shown in Figure 2. For example, suppose that
five units are put under test. Three units fail and their observed times-to-failure (in
hours) are 65, 76, and 84. The last two units are still operating when the test is stopped
at 85 and 100 hours respectively. Therefore, the last two units are considered to be
suspended or right censored.
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Figure 2: Right Censored |
Interval Censored Data
Another type of censored data, shown in Figure 3, is called interval
censored data or inspection data. Interval censored data contains uncertainty as to when
the units actually failed. For example, if five units under test are inspected every 100
hours, then the status of each unit (failed or still running) is known only at the time of
each inspection. If a unit fails, it is known only that it failed between inspections and
the exact time of failure is not known. Instead of an exact time-to-failure, an interval
of time (e.g., between 100 and 200 hours) would be recorded.
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Figure 3: Interval Censored |
Left Censored Data
Left censored data is demonstrated in Figure 4. Left censored data
is a special case of interval censored data in which the time-to-failure for a particular
unit is known to occur between time zero and some inspection time. For example, if the
inspection occurs at 100 hours, a failed unit could have failed at any time between 0 and
100 hours.
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Figure 4: Left Censored |
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