The exact expressions for the convolutions of gamma distributions with different scale parameters is quite complicated. The approximation by means of another gamma distribution is shown to be remarkably accurate for wide ranges of the parameter values, especially if more than two random...

To model demand, the normal distribution is by far the most popular; the disadvantage that it takes negative values is taken for granted. This paper proposes two modifications of the normal distribution, both taking non-negative values only. Safety factors and order-up-to-levels for the familiar...