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Biological SciencesBiology

The Logarithmic Poisson Gamma Distribution: A Model for Leadtime Demand

Authors: Steven Nahmias; W. Steven Demmy; DECISION SYSTEMS BEAVERCREEK OH
 
Abstract: Many of the inventory models which are used in practice rely upon knowning the probability distribution of demand over a leadtime. The common assumption is that this distribution is normal. However, in certain circumstances, the normality assumption may be inappropriate. We consider a process in which demand occurrences obey a stationary Poisson process and at each occurrence a random number of units are demanded. Furthermore, the leadtime for replenishment is assumed to be random as well. Assuming a logarithmic compounding distribution and a gamma distribution of leadtime we derive the exact distribution of total demand in a leadtime. We call this the Logarithmic- Poisson-Gamma (LPG) distribution. An approximation is derived which is equivalent to a scaled version of the negative binomial distribution. In the final section we derive the first four central moments.

Description: Interim rept.
Pages: 19
Report Date: DEC 1981
Contract Number: F33600-80-C-0530
Report Number: A045421

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Keywords relating to this report:
*INVENTORY ANALYSIS
*LEAD TIME
*LOGARITHM FUNCTIONS
*Probability distribution functions
APPROXIMATION_MATHEMATICS_
BINOMIALS
COMPARISON
MATHEMATICAL PREDICTION
OBSERVATION
POISSON EQUATION
RANDOM VARIABLES
STATIONARY
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