A form of independent demand management model in which an order is placed every ‘n’ time units. The order quantity is variable and essentially replaces the items consumed during the current time period. Let ‘M’ be the maximum inventory desired at any time, and let x be the quantity on hand at the time the order is placed. Then, in the simplest model, the order quantity will be M x. The quantity M must be large enough to cover the maximum expected demand during the lead time plus a review interval. The order quantity model becomes more complicated whenever the replenishment lead time exceeds the review interval, because outstanding orders then have to be factored into the equation. These reorder systems are sometimes called fixed-interval order systems, order level systems, or periodic review systems. Synonyms: Fixed- Interval Order System, Fixed-Order Quantity System, Order Level System, Periodic Review System, Time-Based Order System. Also see: Fixed Reorder Quantity Inventory Model, Hybrid Inventory System, Independent Demand Item Management Models,