John D.C. Little is an MIT professor and operations research pioneer, best known for Little's Law: L = λW, where the average number of items in a queuing system equals the arrival rate multiplied by the average time an item spends in the system.
Little's Law is one of the mathematical cornerstones of Reinertsen's queueing-theory-applied framework. Reinertsen uses Little's Law to demonstrate why reducing WIP reduces cycle time — one of the most counterintuitive and consequential insights in principles-of-product-development-flow. The relationship is direct: if you hold arrival rate constant and reduce the number of items in the system (WIP), average time in system must decrease. This provides the rigorous mathematical justification for wip-constraints and batch-size-reduction. Little's Law gives Reinertsen's prescriptions the force of mathematical proof rather than mere assertion — it is not merely advisable to reduce WIP, it is mathematically necessary that doing so reduces cycle time under steady-state conditions.