Slot Assignment

Slot Assignment (also Slotting, Storage Assignment) is the OR problem of deciding which storage location (shelf, bin, slot) each SKU occupies in a warehouse. Objective: minimise expected pick walking distance, balance ergonomic load (heavy SKUs at waist height, light SKUs on top shelves), and group co-ordered SKUs side by side. Classical methods: (1) **ABC class-based assignment** — sort SKUs into A/B/C by pick velocity, place A in aisles closest to the door; (2) **correlated assignment** — popularised by Frazelle (2002), places SKU pairs that appear in the same order side by side; opens a 25-40% gap over plain ABC; (3) **SKU-shelf MIP** — assignment variables plus affinity weight plus ergonomic constraints, solvable directly in small-to-mid warehouses; (4) **simulated annealing / tabu search** — for large scale (100K+ SKUs). Slot Assignment must be considered jointly with Order Picking Routing — solved together they cut total pick time by 25-40%. Canonical survey: de Koster, Le-Duc and Roodbergen (2007); practical reference: Frazelle (2002).