Lead Time Variability: The Hidden Driver of Inventory Risk
When people talk about supplier performance, lead time usually comes up first. Days from order to delivery. It’s easy to track and most procurement teams watch it closely.
The problem is lead time on its own doesn’t tell you much. A supplier averaging 10 days sounds fine. But is that 10 days reliable, or does it swing between 5 and 20? Both suppliers look the same on a dashboard. They are not the same to work with.
Why variability matters more than speed
Safety stock, reorder points, inventory buffers, none of these are built around averages. They’re built around uncertainty. The more unpredictable a supplier is, the more stock you hold in case they’re late. Holding stock means your cash is tied up, you pay for storage space, and you risk losses if demand shifts and items don’t sell.
A predictable supplier lets you plan tightly. An unpredictable one forces you to plan defensively even when the average is the same.
Two suppliers delivering the same item:
| Supplier | Average lead time | Range | How planning feels |
|---|---|---|---|
| A | 12 days | 11–13 days | Easy. Order when stock hits the reorder point. |
| B | 10 days | 4–18 days | Stressful. Hold extra stock for the worst case. |
On paper Supplier B looks better. Faster on average. In practice Supplier A is almost always easier and cheaper to work with, because the planner knows what’s coming.
What variability actually costs
A few things happen when a supplier is unpredictable. The buyer holds more safety stock. The planner spends time chasing orders and asking for updates. Expedited shipping creeps in because people get nervous. Production slows down sometimes. A customer order gets missed. None of this shows up on a lead time report.
The safety stock formula uses the standard deviation of lead time, not the average. Double the variability, double the safety stock. Textbooks have been saying this for decades.
How to see it in your own data
Any procurement team with a purchase order history can work this out. For each supplier, pull every order for the last 12 months and record the lead time in days. Then calculate:
- The average lead time
- The standard deviation of lead time
- The minimum and maximum
Suppliers with a low standard deviation relative to their average are reliable. Suppliers with a high standard deviation are the ones creating hidden cost.
A lot of teams never do this. They look at average lead time, or they measure on time in full, but they rarely look at the shape of the variation itself. It’s one of the cheapest analytical wins in procurement.
What to do about it
- Share the numbers with the supplier. A lot of them genuinely don’t know their variance looks bad, and showing them the data tends to change the conversation.
- Put variability on the scorecard. If you don’t measure consistency you can’t reward it.
- When you’re choosing between two suppliers, don’t just compare averages. Ask for historical data, or run a trial and collect your own.
- A slightly slower but more reliable supplier might cost you less overall once you factor in inventory and admin time. Worth checking before you pick the faster one on paper.
The takeaway
Faster isn’t always better. Consistent almost always is. A supplier who delivers in 10 days every time is worth more to a planner than one who sometimes delivers in 5 and sometimes in 20.
Lead time is the number people see. Lead time variability is the number that drives cost.
Louis Angelus 