To ensure that your results are valid, consider the following guidelines when you collect data, perform the analysis, and interpret your results.
- The data that you collect are usually failure times
- For example, you might collect failure times for units that are tested at a specific temperature. You might also collect samples of failure times under different temperatures, or under different combinations of stress variables. Alternatively, the data might measure product usage in units other than time, such as the amount of mileage for a tire until it fails.
- Your data must be right censored or have no censoring
- Your data should consist of only exact failure times and/or right-censored observations. Data are right censored if some test units do not fail before the study is over, so you know only the time after which the failure occurred. If your data include left-censored observations (you know only the time before which the failure occurred) or interval-censored observations (you know only the times between which the failure occurred), use Nonparametric Distribution Analysis (Arbitrary Censoring). For more information on censored data, go to Data censoring.
- If your data are multiply censored, you must have a column of censoring indicators
- If your test units are censored at different times, your data is multiply censored. With multiply-censored data, failure times are intermixed with censoring times. In that case, you must have a column with text or numeric values that indicate whether each observation was an actual failure or a unit that was removed from the test before failure (censored). For more information on censored data, go to Data censoring.
- To evaluate each cause of failure separately, record failure modes
- A failure mode indicates the cause of a failure. Because different failure modes often have different failure distributions, consider grouping the data by failure mode, when possible. To obtain results for each failure mode, as well as for the overall system, record the cause of each system failure in a worksheet column. For more information, go to What is a failure mode?.