Little’s Law Is Simple. Using It Well Is Not.
WIP, throughput and cycle time form one of the most useful relationships in operations—but only when boundaries and averages are defined correctly.
The relationship
For a stable system over a sufficiently representative period, average WIP equals average throughput multiplied by average flow time: L = λW. The equation is compact enough to memorize and powerful enough to expose contradictions in operating narratives.
If a factory says throughput is unchanged, WIP has doubled and cycle time has not changed, at least one measurement boundary or assumption deserves inspection.
Boundary discipline
The system boundary must be consistent. Counting WIP from release while measuring cycle time only after a downstream milestone mixes definitions. Rework, scrapped entities and changing product mix can also distort casual interpretations.
Little’s Law does not explain causality by itself. It is a relationship among long-run averages. Pair it with process knowledge, queueing theory and variability analysis to understand why the state exists.
A diagnostic habit
Use the equation as a conservation check before reaching for sophisticated models. Good industrial engineering often begins by making the basic accounting of flow internally consistent.