The Little’s Law: Average Lead time = Average WIP / Average Delivery Rate
WIP (Work in Progress) – a number of items in the system overall or in certain stages (columns on the Kanban board).
To use Little’s Law it is important to meet several requirements:
- items entering and leaving the Kanban system are of the same type
- the lead time of items (tickets) in the process is not increasing or decreasing (the system is stable)
- The WIP is fluctuating consistently between a max limit and a lower but consistent value
- all the tickets, that entered the system, must get to its end (no bounced tickets, or tickets that get stuck inside forever).
That is why using Little’s Law may be a bit tricky: even if it looks simple and obvious, you can only use it with a thin tail lead time distribution (Maturity Level 3-4 according to Kanban Maturity Model). The other pre-requisites are also not typical of a lower maturity implementation. WIP limits, a stable system, and a negligible amount of aborted or abandoned tickets – are all attributes or a maturity level 3 or 4 implementations. Little´s Law lures us in with its simplicity. It looks attractive to low maturity organizations with rudimentary Kanban flow systems, but there is danger in using it for forecasts when the prerequisite conditions are not met.
However, if applied correctly the Little’s Law formula has proven its usefulness to understand how flow systems (Kanban systems) behave when WIP changes. Therefore, the benefit of trimming the tail is that it enables you to use Little´s Law as a convenient forecasting technique.
It is important to understand that Little´s Law is a function of averages. That is why it is necessary to be able to calculate an average for each variable, with a reasonably small sample data set. We believe that the threshold for a reasonable sample is 70 to 100 data points (tickets). This will give an opportunity to forecast an average with less than 10% error.
The Little’s Law formula communicates the abilities of the system – the more tickets/tasks you will put in it – the longer your lead time will be. WIP limits encourage you to work on the tickets/tasks one-by-one reducing multi-tasking. The biggest benefit of WIP limits is that tickets spend less time waiting, and less waiting time results in a thinner tail. WIP limits stabilize the system and hence, stable upper and lower limits of variation, that are being necessary pre-requisites for the Little´s Law equation. It is unwise to use Little´s Law if you don´t have WIP controls in place.
How to use the Little’s Law:
You can use Little´s Law for basic forecasting to define how long the current WIP will take until its ready given the previous experience. For instance, if it took 20 days to complete 10 work items, using the formula you can predict how many days (on average) you will need to complete 12 WIP items. Under most, but not all, circumstances, Little´s Law behaves linearly, and hence if 10 items took 20 days, 12 items will probably take 24 days. The equation tends to be more predictable and linear when modeling increases in WIP rather than decreases. The very technical mathematical reasons for this are probably best explained another time, in another place, such as a book on Enterprise Services Planning.