Glossary
Little's Law
Little's theorem, queueing law
Little's law ties together three quantities of any working system: the lead time of a task equals the amount of work in progress divided by throughput. Starting more work in parallel without finishing faster lengthens every single task.
John D. C. Little proved the relationship for queues in 1961. It holds regardless of how work is prioritised, how long individual activities take, or how unevenly requests arrive. That makes it robust for projects.
The formula is:
Lead time = work in progress ÷ throughput
Work in progress (WIP) is the number of tasks started but not finished. Throughput is the number of tasks completed per unit of time. Of the three quantities, throughput can hardly be raised in the short term, while WIP can be cut immediately. That makes limiting open work the only lever that acts without additional capacity.
Same capacity, two ways of working
A team completes 2 tasks per week. Bar length is the lead time of one task.
4 tasksat once
Lead time 2 weeks. The first four results are ready after week 2, the other four follow by week 4.
8 tasksat once
Lead time 4 weeks. All eight results arrive at the very end; nothing is usable before that.
WIP ÷ throughput
The formula assumes constant throughput. In practice it falls as WIP rises, because setup and re-entry time follow every context switch. The effect described therefore shows up more strongly in reality than the calculation suggests.
The limit is implemented as a fixed ceiling of open tasks, in agile settings as a WIP limit per column or per person. In a classic schedule the equivalent is the number of concurrently running activities and the utilisation of the resources involved. Two parallel bars are only genuinely parallel if two people can work on them.
