Reorder points and service level safety stock
Per product and store reorder points and order-up-to levels that turn a demand forecast, the delivery lead time, and a planner's service level target into an order quantity.
Part of the recommended approachTechnique: Rules Engine
Reorder points are the mechanism that actually places replenishment orders, and they stay in place whatever forecast feeds them.
For each product in each store they compare on-hand plus on-order stock with the demand expected before the next delivery can arrive, and order up to a level that covers it with a safety buffer.
The buffer comes from the planner’s service level target and the uncertainty of demand over the lead time, so planners steer the system by category instead of by order.
This is the right place to start on any range, because it is explainable to store staff and it encodes case packs and shelf capacity that no model knows about.
It is the wrong tool on its own for products whose demand is driven by promotions or events, since a reorder point reacts to stock falling and cannot anticipate a spike it has not seen.
It also cannot fix an empty shelf the record believes is full, which is why phantom inventory screening has to run beside it.
Strengths for this problem
- Every order can be explained to a store manager in terms of on-hand stock, the reorder point, and the next delivery.
- Enforces the physical constraints a forecast ignores, such as case pack, minimum presentation stock, and shelf capacity.
- Lets planners change behaviour for a whole category by changing its service level target, instead of editing thousands of orders.
Limitations for this problem
- Only as good as the forecast and the on-hand record it is given, so it reorders nothing when the record shows phantom stock.
- Safety stock formulas assume demand roughly follows a normal distribution, which breaks for slow sellers that sell zero most weeks.
- Static parameters drift out of date when a product's demand, lead time, or shelf space changes and nobody reviews them.
Data needed
- Per product and store: on-hand quantity, on-order quantity, case pack, shelf capacity, and minimum presentation stock.
- Supplier or distribution centre lead time and the store delivery schedule.
- Forecast demand over the lead time and review period, and its uncertainty.
- Service level target per category, set by the planner.
How it is measured
- On-shelf availability per category against its service level target, measured by shelf audit rather than system records.
- Days of supply and backroom overstock per category.
- Share of orders capped or rounded by case pack or shelf capacity, which shows where physical constraints rather than demand set the order.
- Planner override rate per category.
Pitfalls
- Setting every category to the same high service level target, which inflates stock on slow sellers far more than it improves availability.
- Leaving reorder points at their launch values for years while demand, lead times, and shelf space all change.
- Computing safety stock from raw sales variance, which treats stockout zeros as real demand variability.
Human oversight
The planner sets the service level target per category, reviews parameter changes for high-value or perishable products, and approves overrides, which are logged with a reason.