When to use it
Use it with explicit state definitions and reliable entry and exit timestamps.
When to be cautious
A widening band suggests accumulation, not an explanation of its cause; batching and inconsistent data can mislead.
Apply it
WIP at time t = cumulative arrivals − cumulative departures
Use the same system boundary and include all work items consistently. Changes of workflow definition can make trends incomparable.
- Define workflow states and count items consistently.
- Track cumulative arrivals and departures over time.
- Investigate queues and agree a capacity-aware work-in-progress limit.
An example in practice
Testing accumulates while development remains busy. Reduce new starts and resolve the testing bottleneck before adding more unfinished work.
Source & credit
John D. C. Little, A Proof for the Queuing Formula: L = λW (1961).
John D. C. Little (1961): original proof
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