In this formula means Y the supply level during one year, Y1 (second Y ) the average supply, and n-1 the degrees of freedom. This degrees are corresponding with the kind of collection in this calculation. If the total collection is relevant, the degree n is actual. With mathematics of a part of the collection the degree n-1 as above is substituted. The result is a standard deviation as known as a deviation for supply level. If a so called 'spread degree' is wanted from 3Q , must the result be multiplied by 3. This result must be counted up by the average supply level to get in this way a service degree of 99.7%. With a spread degree of 2Q it is possible to realize a service degree of 95.4% and 1Q gives a degree of 68,2%. This degree of service is the percentage of YES-sales if the average limit is already past.
 
 

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