Little’s law is a simple equation explaining how Waiting Time, Throughput and Inventory are related.
Wait Time = Inventory (or WIP) / Throughput
Here is a video about Little’s law:
Fine, what is Little’s law good for?
Well, if a process lead time is too long, chances are that work-in-progress (WIP) is too high. For a given processing rate (Throughput), the lead time will be equal to WIP/Throughput. To reduce the lead time the process Throughput must be increased or the WIP reduced.
Throughput is usually limited by some constraint: machining speed, resources available and so on. It may not be easy to increase Throughput.
WIP on the other hand can generally be controlled by limiting the inventory or the work to be done entering the system upfront.
In this video, Philip Marris explains how to reduce WIP by controlling the flow of work entering the system. Even he does not mention Little’s law, it is indeed used to reduce Inventory.