The model addresses the minimization of a linear combination of the average picking effort, W, and the order cycle time, T (7). The former is computed as the sum, while the latter as the maximum of the individual pickers times ( (8) and (9)). Each item must be assigned to exactly one zone (10), in such a way that the capacity constraints are respected (11). Equality (12) states that the picking time of individual pickers is composed of zone access times and unit item retrieval times. Constraint (13) describes how the expected number of items to be picked is computed for individual pickers. Finally, inequalities ( (14), (15) and (16)) encode the saturated sum approximation of the probability of having to access a given zone in a given milkrun cycle. Parameter M is a sufficiently large number, e.g.