Explain Little's Law and its application in Kanban

What's really being asked

This question tests if you understand the mathematical foundation of Kanban and flow-based systems. It's not just about knowing the formula, but about using it to reason about system design trade-offs. The interviewer wants to see if you can connect team-level decisions, like setting WIP limits, to business-level outcomes, like predictable delivery times. It separates candidates who just move tickets from those who design and manage a predictable system.

The full answer

A strong answer has three parts. First, state the formula clearly: Average Lead Time = Average Work In Progress (WIP) / Average Throughput. Second, explain the direct relationship and trade-offs. For example, if WIP increases but throughput stays the same, lead time will increase. Conversely, to decrease lead time, you must either decrease WIP or increase throughput. Third, provide a practical application, like using the law to calculate a target WIP limit. If a team has a throughput of 10 items per week and wants to achieve a 1-week lead time, their target WIP should be 10 items.

The mistakes people make

A major red flag is reciting the formula without mentioning its critical prerequisite: the system must be stable. Little's Law only holds true for systems where the arrival rate and completion rate are roughly equal over time. Applying it to a chaotic, constantly overloaded system will produce misleading numbers. Another mistake is assuming you can just increase throughput by pushing more work in; Little's Law shows this will actually increase lead time if not managed, as more work increases queue times.

What usually comes next

Be ready for "How would you gather the data for these metrics in a real-world team?" or "What do you do if your system isn't stable? How do you get it there?" Another common one is, "Walk me through an example where you used this to convince a product manager to limit new work."

A concrete example

A team has an average of 20 items in progress (WIP). They complete an average of 4 items per week (Throughput). Using Little's Law, their average lead time is 20 / 4 = 5 weeks. If stakeholders want a 2-week lead time, the team can't just work harder. They must either increase throughput to 10 items/week (often unrealistic) or reduce their WIP to 8 items (4 items/week * 2 weeks). This gives them a concrete, data-backed way to justify WIP limits to management.