premise: To determine the optimal quantity for a SKU, the retailer finds the number of units to order so that the expected marginal cost of stocking an additional unit and not being able to sell it equals the expected marginal cost of not stocking that unit when it would have sold if available.
hypothesis: Optimal quantity for a SKU is determined by the retailer, so the marginal cost of stocking a unit and how it sells equals the expected marginal cost.