If they are treated as random variable, they are called probabilistic or stochastic. The probabilistic model assume that the average demand remains approximately constant with time and that it is possible to state the probability distribution of the demand. In particular, since the lead time is the usual period of concern, attention is focused on the distribution of demand during the lead time. Lead time demand is a random variable if at least one of its primary components (demand and/or lead time) is a random variable.