When to order-Material control
If deliveries from suppliers normally take two weeks to arrive, then replenishment orders should be placed with them when the level of stocks represents two weeks' supply. For illustration, if usage is 200 units a week, an order (the EOQ) will be placed when the stock level falls to 400 units. Figure below illustrates that, with certain knowledge of usage and lead time, delivery takes place just as stock is exhausted.
Figure: Stock levels when usage and replenishment times are known
Figure: Stock levels when usage and replenishment are uncertainLead times and usage may not be stable and provision against running out of stock becomes necessary figure shown above Safety stocks have a cost, however, and this has to be balanced against the cost of running out of stock. `Stock outs' may cause loss of customers and the probability of this happening at various levels of safety stock must be estimated. The point, at which the cost of carrying safety stocks plus the cost of `stock outs' is lowest, indicates the safety stock level. Notice that uncertainty causes the reorder level to be at a higher level to include the required safety stock.