What it means
Reorder points, production schedules and sales promises depend on how long supply takes. A buyer may still use a ten-day lead time because it is written in a contract or master-data field, while recent orders routinely take 16 days, and the mismatch can cause stockouts even if the purchasing team follows its process exactly.
Drift analysis checks whether the planning number still reflects reality. Choose clear start and end dates, since lead time might run from purchase order issue, supplier acknowledgement or deposit receipt to dispatch, arrival or usable stock after inspection, and different definitions answer different questions.
For inventory planning, the time until usable stock is often what matters, so record supplier production, transport, customs and internal receiving separately when possible to make the cause visible. Compare like with like, because a custom item will not have the same lead time as a standard spare and order quantity, season, delivery destination and shipment method matter.
A few urgent air-freight orders should not make normal sea-freight lead time appear shorter, so use a rolling median or distribution alongside the average to see variability and long tails. Confirm source dates rather than relying on manually entered promised dates that are revised after each delay.
Record the acknowledgement date and any revisions, because a supplier that consistently promises a date it cannot meet needs a different plan even if its stated contractual lead time remains unchanged. Investigate causes with the supplier, since capacity may be tight, raw materials late, approvals slow or transport unreliable, and the buyer may contribute through late specifications or changes to order quantities.
Respond before customers suffer by updating the planning lead time and safety stock where justified, placing orders earlier, qualifying another source or revising a delivery promise. Each response has cost, because holding more stock ties up cash, expediting costs money and changing supplier takes time.
Do not simply inflate lead time so much that every order appears on time while stock grows unnecessarily, and revisit the model when performance improves. Keep commercial and operational measures distinct, since a supplier may miss a contractual commitment and still be predictable at a longer lead time.
The buyer can pursue agreed remedies while planning with actual performance. Conversely, early deliveries can create storage pressure if the business is not ready, so look at service reliability as well as speed.
For owners, drift is an early warning that a quiet master-data number is no longer true. Correcting it can prevent repeated emergency purchases and broken customer commitments.
In practice
Real-world examples.
Example
A supplier's median usable-stock lead time rises from 14 to 21 days over three months. The buyer reviews reorder points and the supplier's capacity.
Example
A parts vendor ships on time, but inspection adds five days because documentation is incomplete. The business separates supplier and internal delays.
Example
A seasonal peak makes transport slower each November, so the buyer plans earlier rather than treating every late delivery as a surprise.
Formula
Calculation
Lead time drift = Observed rolling lead time - Lead time used in the current plan
Worked example. An invented buyer uses a 12-day planning lead time, while recent comparable orders take a median 17 days to become usable.
- Drift = 17 - 12 = 5 days longer than plan.
- The team tests whether the extra five days are recurring before changing reorder rules and customer estimates.
Show variability too: a median of 17 days with occasional 30-day orders may still need a buffer.Case study
Seen in the real world.
This illustrative and entirely fictional example follows Grove Medical, an invented clinic-supplies distributor. Its purchasing system used a 10-day lead time for a common consumable. Over several months the supplier's deliveries slowed, but buyers manually expedited each shortage without updating the field. Sales kept promising customers the old delivery window. Grove reviewed comparable purchase orders from acknowledgement to inspection release.
The median had moved to 16 days and the longest deliveries were over 20. The supplier described a capacity constraint and gave a credible recovery plan. Grove increased short-term buffer stock for critical customers, revised new promises and began qualifying a second source. It retained the contractual performance record separately. Emergency freight fell after the planning value matched observed supply more closely.
Watch out
Common mistakes.
- Comparing unlike products or shipping methods and treating the result as one supplier-wide lead time.
- Letting revised promised dates replace original dates, hiding the pattern of delay.
- Updating the planning value without considering variability, inventory cost and supplier remedies.
Questions
People also ask.
Is one late delivery evidence of drift?
Not usually. Look for a sustained pattern across comparable orders and investigate outliers separately.
Which end date should be used?
Use the point relevant to the decision, often when stock is usable, while showing transport and receiving stages separately.
What should change when drift is confirmed?
Review reorder rules, supplier action, alternative sources and customer promises, with the cost of extra buffer stock visible.
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