Littles Law (Lean Six Sigma, by M. George, 2002, McGraw Hill) is used to calculate the Process Lead Time by dividing the number of items in process by the completions per hour through this process lead time formula.
Littles Law: Lead Time=# Items In-Process / Completion Rate
For example, if it takes two hours on average to complete each Purchase Order, then there are 0.5 completions per hour. This is the denominator of the Littles Law equation. If there are ten Purchase Orders waiting in queue (the numerator), then Littles Law says we need ten divided by one-half equals twenty hours lead time for the process. In other words, we cannot process any new orders until the twenty hour lead time has allowed the existing Work in Process to be completed.
Once Lead Time is known, Velocity can be calculated and analyzed.
Learn more about the Lean Six Sigma principles and tools for process excellence in Six Sigma Demystified (2011, McGraw-Hill) by Paul Keller, in his online Lean Six Sigma DMAIC short course ($249), or his online Green Belt certification course ($499).