Continuous vs. Periodic Review: Why CPG Lines Reorder on a Clock, Not a Trigger

Here is a frustration nearly every scheduler has lived through. You set a clean, textbook reorder point for each SKU — forecast usage over the lead time, plus a safety-stock cushion. The math is right. And you still eat stockouts on some SKUs and drown in excess on others.

The reorder point isn't broken. Your assumption about it is. A reorder point quietly assumes one thing: that you can start replenishing the instant inventory crosses the trigger. On a dedicated line with a supplier standing by, maybe. On a shared CPG line that rotates through a dozen SKUs behind sequence-dependent changeovers, absolutely not. You can only make each SKU when the line comes back around to it.

That single fact changes which inventory-control policy actually describes your floor — and how much safety stock you truly need. Let's walk through the two foundational policies, the hybrid you're probably already running, and the hidden cost the trigger model hides.

The two policies, defined plainly

There are two classic ways to control replenishment, and almost everything else is a variation on them.

Continuous (perpetual) review — the (s, Q) policy. You monitor inventory continuously. The moment it drops to the reorder point s, you order a fixed quantity Q. The reorder point is calculated as forecast demand over the replenishment lead time plus safety stock. Because real lead time is never zero, the reorder point always sits above zero, so replenishment arrives before you run dry (Reorder point, Wikipedia). This is trigger logic: watch the line, cross the line, act.

Periodic review — the (R, S) policy. You check inventory every R periods — say, every Monday, or every time the production wheel comes back around. When you check, you order enough to bring the inventory position back up to a target level S, the order-up-to level. This is grounded in the base-stock (order-up-to) model, where inventory position is driven back up to a base-stock level on each review, and it connects directly to the newsvendor model when there's a single replenishment (Base stock model, Wikipedia). This is clock logic: check on a cadence, top up to target.

Picture the two sawtooth charts. Under continuous review, every tooth is the same height (you always order Q) but the timing varies with demand. Under periodic review, the timing is fixed (every R) but the order size varies — you buy back whatever you consumed since the last check. That difference is the whole story.

The Min/Max hybrid you're probably already running

Most CPG planners don't consciously pick (s, Q) or (R, S). They run Min/Max, which blends both. The minimum is the reorder trigger; the maximum is the order-up-to target. A Min/Max system reacts to demand using sales data and auto-generates a replenishment order at the right time, and it scales well across businesses carrying a wide range of SKUs (Netstock).

That's the good news. The trap is that Min/Max values are usually set once and forgotten. Under ongoing supply and demand disruption, static EOQ and Min/Max settings can put the supply chain at risk — the parameters have to be dynamic, recalculated as demand and lead times move (Netstock). A Min that was right last spring can be a stockout machine after a demand shift.

The hidden cost of the review gap

Here's the section that costs — or saves — you real money.

Under continuous review, safety stock only has to protect against demand variability over the lead time. You're watching every second, so the only window of exposure is the time between placing an order and receiving it.

Under periodic review, safety stock has to protect against variability over the review period plus the lead time — (R + L). Why? Because between checks, you're blind. If demand spikes the day after you review, you can't respond until the next review R periods away, and then you still wait out the lead time L. The exposure window is the whole gap.

That means periodic review is structurally more expensive in safety stock. A worked mini-example makes it concrete. Say a SKU sells 100 units/day with a daily demand standard deviation of 30 units, and lead time is 4 days.

• Continuous review covers 4 days. Safety stock scales with the square root of the coverage window: √4 = 2. So safety stock ≈ z × 30 × 2 = 60z units.
• Periodic review with a 7-day review period covers R + L = 11 days. √11 ≈ 3.32. Safety stock ≈ z × 30 × 3.32 ≈ 100z units.

Same service target, same demand — but the periodic policy needs roughly 65% more safety stock purely because of the review gap. That extra inventory isn't waste from bad planning; it's the honest price of only being able to check and act on a cadence. Continuous-review math simply hides that cost because it pretends the gap doesn't exist.

The how-much-to-order leg has its own limits. The classic economic order quantity, EOQ = √(2·D·S/H), gives you a batch size — but it isn't dynamic and assumes stable demand, ordering, and holding costs, which makes it a poor fit for seasonal or lumpy CPG demand (Netstock). For the production side of that question, see our post on economic production quantity and optimal batch size and dynamic lot-sizing for lumpy demand.

Why CPG lines are periodic whether you model them that way or not

This is the core insight. A shared line with changeovers can only touch each SKU on a rotation. You produce SKU A, change over, run SKU B, change over, and so on — and SKU A doesn't come back around until the wheel completes. That interval between production opportunities has a name: EPEI, every product every interval, the production wheel we cover in EPEI: every product every interval.

Here's the punchline: your EPEI interval is your review period. You cannot act the instant a SKU crosses its reorder point, because the line is busy running something else. The effective review period equals the time between production opportunities for that SKU. So no matter what policy you wrote down, your floor behaves like a periodic-review system.

That's why the continuous-review reorder point disappoints. It sizes safety stock for lead time alone, but your real exposure is EPEI + lead time. You've been modeling a trigger you can't pull.

The two decisions are cleanly separable, and thinking of them that way clarifies everything:

• The production rotation (EPEI, changeover sequence, time fences) sets your review period R.
• The inventory policy sets your trigger and target quantities s and S around that cadence.

Sequencing tightens R. If you order runs to minimize changeover time — see sequence-dependent changeover scheduling — you shorten the wheel, which shrinks the review gap, which directly lowers the safety stock every SKU on the line needs to carry. Faster rotation is cheaper inventory.

Push/pull sidebar. Review cadence also interacts with how you trigger production. A pull system replenishes what was consumed; a push system builds to forecast. Your review period shapes which is even feasible for a given SKU (Netstock on push vs. pull). High-turn, steady items can pull on a tight wheel; erratic, slow movers usually need a forecast-driven push with a fatter buffer.

A decision guide: which policy per SKU

You don't pick one policy for the whole plant. You segment. The ABC-XYZ lens is the natural filter: ABC by value, XYZ by demand variability.

SKU profile	Recommended policy	What drives safety stock
A-item, steady (AX)	Tight order-up-to synced to a short wheel; monitor closely	Low variability over (R+L); keep R short
A-item, erratic (AZ)	Periodic order-up-to with high service target	Demand variability dominates; buffer the review gap
B/C-item, steady (BX/CX)	Standard periodic order-up-to on the wheel	Modest buffer over (R+L)
Long-tail, intermittent (CZ)	Periodic, forecast with intermittent-demand methods	Lumpy demand; see below

For that intermittent long tail, ordinary averages mislead you — use Croston's method for slow-moving SKUs to forecast before you set the order-up-to target. And decide your service target deliberately using cycle service level vs. fill rate, since fill rate is the probability an order doesn't cause a backorder (Base stock model) and it drives the z in your safety-stock math.

Setting the parameters: an operator checklist

1. Derive the review period from EPEI, not wishful thinking. Measure how often the line actually returns to each SKU. That real interval is R. Don't assume you can respond daily if the wheel is weekly.
2. *Size safety stock over (R + L), not L alone.* This is the single most common miss. Cover the full exposure window between production opportunities plus the lead time.
3. Make the parameters dynamic. Recompute Min/Max and safety stock when demand variability or lead time shifts. Static settings are a known disruption risk (Netstock best practices).
4. Sanity-check the order-up-to S against reality. Does the target quantity fit the line's capacity in one run? Does it exceed shelf life before it can be consumed? See shelf-life-constrained scheduling — a high S on a perishable SKU trades stockouts for spoilage.

Common failure modes

• Continuous-review reorder point on a line that can't respond continuously. The trigger fires, the line is mid-run on another SKU, and you're chronically late. The fix is to accept the periodic reality and size for the gap.
• Static Min/Max, never revisited. Values set at launch and left alone become wrong the moment demand shifts. Schedule a recompute cadence.
• Safety stock sized for lead time only. The default in most textbooks and many ERP defaults — and it silently underprotects every SKU on a shared line by ignoring the review gap.
• Chasing a tighter wheel with the same buffers. If you shorten R through better sequencing, lower your safety stock to bank the savings; otherwise you're paying for protection you no longer need.

Key takeaways

• Continuous review (s, Q) watches inventory and orders a fixed quantity at a trigger; periodic review (R, S) checks on a cadence and tops up to a target. Min/Max is the practical hybrid.
• A shared CPG line cannot act the instant it crosses a reorder point. Its EPEI rotation is its review period — so it behaves like a periodic-review system whether you model it that way or not.
• Periodic review must cover demand variability over review period + lead time, which is structurally more safety stock than continuous review. The review gap is a real cost the trigger model hides.
• Separate the two decisions: the production rotation sets R; the inventory policy sets s and S. Tightening changeovers and sequencing shrinks R — and directly lowers the safety stock every SKU carries.
• Match your review policy to your actual production cadence, and size safety stock for the gap you truly have. To stop reacting and start planning, see stop firefighting stockouts.

Sources

• Reorder point — Wikipedia
• Base stock model — Wikipedia
• The Definitive Guide to Inventory Ordering — Netstock
• Best Practices for Inventory Replenishment — Netstock
• Push vs. Pull Systems in Manufacturing — Netstock
• Safety stock — Wikipedia