Mean Cumulative Function Plot
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Use the mean cumulative function plot to visualize whether your system reliability is improving, deteriorating, or staying constant. The mean cumulative function plot displays the mean cumulative function versus time. The plot is a step function with steps at system failures or endpoints of intervals.

When you have interval data, Minitab plots the middle point.

The plot provides information about the pattern of system failures:

·    A straight line indicates that system failures are remaining constant over time

·    A curve that is concave down indicates that the time between failures is increasing over time - your system reliability is improving

·    A curve that is concave up indicates that the time between failures is decreasing over time - your system reliability is deteriorating

Below are examples of mean cumulative function plots for improving, stable, and deteriorating systems.