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:
Where:
M_{cti} = repair time for an individual maintenance action

= 
average repair time for N observations 
S_{Mct } = 

= 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:
where:
z(t _{1} ) = value from normal distribution function corresponding to the percentage point (1) on the maintainability function for which M_{Maxct} is defined.
Values of z(t _{1}) as a function of (1) are shown in Table XIV.
Table XIV  Values of z(t' 1a ) Most Commonly Used in
Maintainability Analysis
(1) 
z(t' _{1}
) 
0.80 0.85 0.90 0.95 0.99 
0.8416 1.036 1.282 1.645 2.326 