What is the cumulative failure rate?

The cumulative number of failures at a particular time divided by the time. For example, if a survivability test had 500 failures by the 5^th hour of the test, the cumulative failure rate at hour 5 would be 100 failures per hour.

In the analysis of a repairable system, the Duane plot is a scatterplot of the cumulative failure rate over time. A negative slope shows reliability improvement, a positive slope shows reliability deterioration, and no slope (a horizontal line) shows a stable system.