In reliability analysis, failure data frequently contain individual times to failure. For example, you might collect times to failure for units operating at a particular temperature. You might also collect samples of times to failure under different temperatures, or under different combinations of stress variables.
The exact time that each item failed is known. For example, an engineer tests electric fans and records the exact time to failure of each fan.
Failures are seen only if they occur before a particular time. A unit surviving longer than that time is considered a right-censored observation. Right-censored data are sometimes time-censored or failure-censored. Time censoring means that you perform the study for a specified period of time. All units still operating at the end of the study are time-censored. Time censoring is also known as Type I censoring on the right. Failure censoring means that you conduct the study until you observe a specified number of failures. Failure censoring is also known as Type II censoring on the right.
For example, suppose that an engineer tests five fan belts. Three fan belts fail in 67 hours, 76 hours, and 104 hours. The remaining two fan belts are still operating when the engineer stops the test at 110 hours. These last two fan belts are right-censored at 110 hours.
Minitab interprets this data set as single censoring because the failure times of the censored items (units 13 - 15) are the same as the failure time of the 12th unit.
Minitab interprets this data set as multiple censoring because the failure times of units 13 - 15, are larger than the failure time of the 12th unit. If you stopped the study after the 12th failure, the subsequent times would not be greater than the time of that last failure. To have Minitab interpret the data as single censoring, you need to enter 33 in the 2nd column for rows 13-15.
Failures occur between two particular times. Interval-censored data contain uncertainty as to when units actually fail.
For example, suppose that instead of recording exactly when ten transistors fail, an engineer inspects them every 12 hours. Therefore, the engineer knows the status of each transistor (failed or still operating) only at the time of each inspection. Instead of exact failure times, the engineer records the data as failure time intervals. So, for example, a transistor may fail between 60 and 72 hours.
Failures occur before a particular time. Left-censored data are a special case of interval-censored data in which failure times occur sometime between zero and an inspection time.
For example, glass capacitors are put on test at high voltage levels to accelerate their failure times. Engineers examine the capacitors every 12 hours to see which have failed. At the first inspections, 2 capacitors have failed. The failure times for these two units are left censored.
Testing all units to failure in a life test usually is not recommended, especially if you are only interested in the lower percentiles of the distribution. You should use time censoring for test plans when you have a specific period of time in mind for your test.
To minimize cost, you need to balance the test duration and sample size. For a particular precision, Minitab displays a list of sample sizes for each censoring time you provide. As time increases, the sample size decreases. Choose the time and sample size combination that minimizes costs.
For an accelerated life test plan, you only need to provide one set of censor times. Each time in the set corresponds to the censor time at a stress level. The first time corresponds to the lowest stress level, the second time corresponds to the second stress level, and so on.
Use failure censoring when you are estimating lower percentiles or when you have limited test positions.