The normal distribution can be used to model the reliability of items that experience wearout failures. Given a mean life, μ and standard deviation, σ, the reliability can be determined at a specific point in time (t).
The normal distribution probability density function, reliability function and hazard rate are given by:
Probability Density Function
For example, given a mean life of a light bulb of μ=900 hours, with a standard deviation of σ=300 hours, the reliability at the t=700 hour point is 0.75, as represented by the green shaded area in the picture below.
The calculation inputs below show units of "hours," but any life metric (cycles, years, etc.) can be used as long as there is consistency among the three inputs.