The mean cumulative function is the average cumulative number of failures or the average cumulative cost over all systems. Use this measure of the cumulative failures (or the cumulative cost) to determine whether repairs (or repair costs) are increasing, decreasing, or remaining constant.

For information on how to calculate the mean cumulative function, see Meeker and Escobar^{1} and Nelson^{2}.

The standard errors are the standard deviations of the MCF. The standard error is a measure of the variability in the estimate.

For information on how to calculate the standard error of the mean cumulative function, see Meeker and Escobar^{1} and Nelson^{2}.

When the MCF is positive, the confidence limits for MCF are calculated as follows:

Term | Description |
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estimated mean cumulative function | |

estimated standard error of the mean cumulative function | |

z_{α} | the (1 + α) / 2 percentile of a standard normal distribution where 100α % is the confidence level |