In maintainability, the normal distribution applies to relatively straightforward maintenance tasks and repair actions (e.g., simple removal and replacement tasks) which consistently require a fixed amount of time to complete. Maintenance task times of this nature are usually normally distributed, producing a probability density function given by:
Mcti = repair time for an individual maintenance action
|average repair time for N observations
||= standard deviation of the distribution of repair times, based on N observations
N = number of observations
The mean time to repair ( M ct ) is given by:
The median time to repair ( Ä M ct ) is given by:
which is equal to the mean time to repair because of the symmetry of the normal distribution.
The maximum time to repair is given by:
z(t 1- ) = value from normal distribution function corresponding to the percentage point (1-) on the maintainability function for which MMaxct is defined.
Values of z(t 1-) as a function of (1-) are shown in Table XIV.
Table XIV - Values of z(t' 1-a ) Most Commonly Used in