To describe product reliability in terms of when the product fails, the cumulative failure plot displays the cumulative percentage of items that fail by a particular time, t. The cumulative failure function represents 1 − survival function.
- The center line is the estimated cumulative failure percentage over time.
- The right line connects the lower bounds for the cumulative failure percentage at each time point. The left line connects the upper bounds for the cumulative failure percentage at each time point.
When you hold your pointer over the curve, Minitab displays the cumulative failure probability and failure time.
Use this plot only when the distribution fits the data adequately. If the distribution fits the data poorly, these estimates will be inaccurate. Use the distribution ID plot, probability plot, and goodness-of-fit measures to determine whether the distribution adequately fits the data.
For the engine windings data, the probability of the engine windings failing by 70 hours at a temperature of 80° C is approximately 60%. The cumulative failure function is based on the lognormal distribution with location = 4.09267 and scale = 0.486216.