Recommended spare stock is calculated interactively (using Poisson distribution) by finding the smallest n for which the following inequality is true:

where        t is a repair turnaround time (TAT) for repairable items, or total expected operating hours during prediction period for discardable items.

 

For very large stocks (λt > 720), the following formula based on normal approximation is used:

where        ZR is a fractile of the cumulative normal distribution corresponding to the required NSP.

Achieved NSP is calculated as the probability corresponding to the fractile ZA of the cumulative normal distribution: