Overview of Parametric Distribution Analysis

Use Parametric Distribution Analysis to estimate the overall reliability of your system when your data follow a parametric distribution.

You can select from 11 distributions to model your lifetime data: smallest extreme value, Weibull, 3-parameter Weibull, exponential, 2-parameter exponential, normal, lognormal, 3-parameter lognormal, logistic, loglogistic, and 3-parameter loglogistic. Based on the fitted distribution, you can do the following:

Display parameter estimates and distribution characteristics, such as mean time to failure (MTTF)
Estimate percentiles, survival probabilities, cumulative failure probabilities, and their confidence intervals
Display survival plots, cumulative failure plots, and hazard plots to graphically analyze the probability of failure or survival
Assess the fit of the selected distribution with a probability plot

Use this analysis for exact failure times or for any censoring scheme.

Exact failure data: Record the failure time at a fixed point in time before the end of the study or reliability test
Right-censored data: Record a time when the unit still survived at the end of the study or reliability test. For example, a patient may quit a study before it ends, or a unit is still operating after the allocated testing time. The exact failure time is unknown but will certainly occur beyond the allocated study time.
Interval-censored data: The exact failure time is unknown, but it occurred in a known interval. For example, a unit was still operating at 12 when the reliability engineer left for a mid-day break. Upon the engineer’s return at 12:30 they noticed that the unit has failed. Thus, the unit failed in the interval starting at 12 and ending at 12:30.
Left-censored data: Left-censored data is a special case of interval-censored data point where the beginning point of the interval may be time 0 or may be unknown. The engineer only knows that the unit failed by some specific time since the beginning of the reliability test.

In this analysis, interval censoring allows for the specification of left-censored data and data that combine different types of censoring. For more information, go to Data censoring.

When your product fails in different ways, you can enter failure mode information for this analysis to evaluate the impact of each type of failure on the overall reliability. Each failure mode is assumed to be independent and may be modeled by different distributions. By analyzing each failure mode individually, you can more easily prioritize your improvement efforts.

Tip

The analysis stores a reliability data definition that includes the sample columns and the censoring scheme. Select the definition to quickly run Distribution ID Plot, Distribution Overview Plot, Parametric Distribution Analysis, and Nonparametric Distribution Analysis on the same data.

Where to find this analysis

To perform a parametric distribution analysis, choose Stat > Reliability/Survival > Distribution Analysis > Parametric Distribution Analysis.

When to use an alternate analysis

To determine which parametric distribution best fits your data, use Distribution ID Plot.
If no parametric distribution fits your data adequately, use Nonparametric Distribution Analysis.