Continuous review, written (s, Q), is the policy that monitors stock position permanently and triggers an order of a fixed quantity Q as soon as the threshold s is crossed. Its defining property is not the monitoring frequency but its economic effect: because you can react at any moment, the window during which you are exposed to uncertainty is limited to the replenishment lead time alone. It is the policy that, at equal service, requires the thinnest buffer.
Why it matters
Once the trigger threshold and the lot size are known, it remains to decide how the system observes stock: permanently, or at regular intervals. The question looks technical. It is in fact structural, because it determines how long the company is exposed to uncertainty without being able to react, and that duration directly drives the capital tied up in safety stock.
Continuous review puts that exposure at its minimum. Since the threshold can be crossed and detected at any moment, the only period of vulnerability is the delivery lead time. Periodic review adds the wait until the next check: if stock falls below the threshold just after a review, nobody will see it before the next one, and you must hold out through that whole interval on top of the lead time.
The financial consequence is direct and can be read off the signature: the buffer grows with the square root of the window covered. Moving from permanent monitoring to a monthly review on a two-week lead time does not cost a few percent of safety stock, but often more than half again. It is one of the rare trade-offs where an organisational choice translates so mechanically into working capital.
That said, continuous review is not superior in all circumstances. It triggers orders at unpredictable moments, which complicates shipment consolidation, route negotiation and coordination with suppliers who prefer a steady rhythm. Many companies willingly pay for a thicker buffer to buy regularity. What matters is that this be a conscious choice, not the default setting of a software package.
The rule to remember fits in one sentence: continuous review protects over the lead time, periodic review over the lead time plus the review interval. The entire extra cost of the second comes from there, and it can be computed in advance.
This is why the policy is chosen by segment, never globally. On high-value, fast-moving references, where every point of stock weighs, permanent monitoring is justified and pays for itself. On the long tail, where tied-up stock is small but the administrative cost of monitoring is the same, a grouped periodic review costs less in total, despite its thicker buffer.
The expert lesson is not to confuse the policy with the tool. Many systems advertise continuous review but rely on a stock position refreshed in batch overnight: the real exposure window is then that of the update cycle, not that of the lead time. The buffer must be computed on the effective window, the one you observe, not the one the configuration claims to offer.
The mechanism
The operation is simple. On every movement, the system updates the stock position, that is, physical stock plus orders already placed minus customer commitments. As long as that position stays above the threshold s, nothing happens. As soon as it reaches it, an order of fixed quantity Q goes out. The threshold answers the timing question, the quantity the volume question.
The two parameters come from distinct reasoning and are computed separately. The threshold is a classic reorder point: expected consumption during the lead time, plus the safety stock matching the target service level. The quantity belongs to the economics of lot sizing, hence to the economic order quantity calculation, rounded to packaging constraints.
The most common variant replaces the fixed quantity with a top-up to a target level, written (s, S) or min-max: instead of always ordering the same volume, you order enough to climb back to a ceiling. This form absorbs irregular demand better, where a sharp drop through the threshold would make a fixed quantity insufficient, but it produces orders of varying size, less convenient for transport.
The decisive technical point remains the window the buffer covers. Under continuous review, safety stock is computed on the lead time alone. Under periodic review, it must cover the lead time plus the interval between checks. Taking a buffer computed for one policy and applying it to the other is a silent error: service degrades without the configuration looking faulty, since the threshold itself appears correct.
The traps
Moving from continuous to periodic review while keeping the same safety stock degrades service invisibly: the window to cover has widened but the buffer has not moved. Stockouts appear months later, with the configuration seemingly not at fault.
A policy advertised as continuous but fed by a stock position refreshed once a night is, economically, a daily review. Until the real update frequency and the reliability of movements are verified, a buffer computed on the lead time alone is undersized.
Choosing the same policy for every reference means over-administering the long tail or under-protecting critical references. Permanent monitoring is justified where stock is expensive; elsewhere, a grouped, regular review is cheaper, thicker buffer included.
The rollout
Establish the real lead time and the effective refresh frequency of the stock position. It is this observed window, not the one the configuration announces, that must underpin the buffer calculation.
Cross value and regularity (ABC-XYZ logic) to decide where permanent monitoring pays for itself and where a grouped periodic review is cheaper, administrative cost included.
A threshold policy is only as good as the data that triggers it. Cycle counts, discrepancy handling, integration of in-transit orders: without that base, the threshold fires wrongly or too late.
Set the threshold from the reorder point and the quantity from the economic calculation, then round to pack sizes. The two parameters answer two distinct questions and cannot be derived from one another.
Test the chosen rhythm against transport and supplier realities. If consolidation demands regularity, adopt it explicitly and finance the matching buffer, rather than enduring poorly consolidated irregular orders.
Neighboring concepts
From knowledge to action
The review rhythm sets the exposure window, hence the capital tied up in buffers. Our Inventory & distribution file arbitrates your policies by segment and resets buffers on the window you actually observe.