This is post 7 of 9 in our Little's Law series.
You may or may not be surprised to hear me say that the Little's Law equation is indeed deterministic. But, as I have mentioned several times in the past, it is not deterministic in the way that you think it is. That is, the law is concerned with looking backward over a time period that has already been completed. It is not about looking forward; that is, is not meant to be used to make deterministic predictions. As Dr. Little himself says about the law, "This is not all bad. It just says that we are in the measurement business, not the forecasting business". (1)
In other words, the fundamental way to NOT use Little's Law is to use it to make a forecast.
Let me explain, as this is a sticking point for many people (again, most interwebs blog posts get this wrong). The "law" part of Little's Law specifies an exact (deterministic) relationship between average WIP, average Cycle Time, and average Throughput, and this "law" part only applies only when you are looking back over historical data. The law is not about—and was never designed for—making deterministic forecasts about the future.
Little's Law wasn't designed for making deterministic forecasts about the future.
For example, let's assume a team that historically has had an average WIP of 20 work items, an average Cycle Time of 5 days, and an average Throughput of 4 items per day. You cannot say that you are going to increase average WIP to 40, keep average Cycle Time constant at 5 days, and magically, Throughput will increase to 8 items per day—even if you add staff to keep the WIP to staff ratio the same in the two instances. You cannot assume that Little's Law will make that prediction. It will not. All Little's Law will say is that an increase in average WIP will result in a change to one or both of average Cycle Time and average Throughput. It will further say that those changes will manifest themselves in ways such that the relationship among all three metrics will still obey that law. But what it does not say is that you can deterministically predict what those changes will be. You have to wait until the end of the time interval you are interested in and look back to apply the law.
The reason for the above is because--as we saw in the last post--it is impossible to know which of Little's assumptions (or how many times) you will violate in the future. As a point of fact, any violation of the assumptions will invalidate the law (regardless of whether you are looking backward or forward).
But that restriction is not fatal. The proper application of Little's Law in our world is to understand the assumptions of the law and to develop process policies that match those assumptions. If the process we operate conforms—or mostly conforms—to all of the assumptions of the law, then we get to a world where we can start to trust the data that we are collecting from our system. It is at this point that our process is probabilistically predictable. Once there, we can start to use something like Monte Carlo simulation on our historical data to make forecasts, and, more importantly, we can have some confidence in the results we get by using that method.
There are other, more fundamental reasons why you do not want to use Little's Law to make forecasts. For one thing, I have hopefully by now beaten home the point that Little's Law is a relationship of averages. I mention this again because even if you could use Little's Law as a forecasting tool (which you cannot), you would not want to, as you would be producing a forecast based on averages. Anytime you hear the word "average," you must immediately think "Flaw of Averages" (2). As a quick reminder, the Flaw of Averages (crudely) states that "plans based on average assumptions will fail on average." So, if you were to forecast using LL, then you would only be right an average amount of the time (in other words, you would most likely be wrong just as often as you were right--that's not very predictable from a forecasting perspective).
Plans based on average assumptions will fail on average
Having said all that, though, there is no reason why you cannot use the law for quick, back-of-the-envelope type estimations about the future. Of course, you can do that. I would not, however, make any commitments, WIP control decisions, staff hiring or firing decisions, or project cost calculations based on this type of calculation alone. I would further say that it is negligent for someone even to suggest doing so. But this simple computation might be useful as a quick gut check to decide if something like a project is worth any further exploration.
While using Little's Law to forecast is a big faux pas, there are other myths that surround it, which we will cover very quickly in the next post in the series.
Little, J. D. C. *Little's Law As Viewed on Its 50th Anniversary* https://people.cs.umass.edu/~emery/classes/cmpsci691st/readings/OS/Littles-Law-50-Years-Later.pdf
Savage, Sam L. *The Flaw of Averages*. John Wiley & Sons, Inc., 2009.
Vacanti, Daniel S. *Actionable Agile Metrics for Predictability* ActionableAgile Press, 2014.
Explore all entries in this series
How NOT to use Little's Law (this article)
About Daniel Vacanti, Guest Writer
Daniel Vacanti is the author of the highly-praised books "When will it be done?" and "Actionable Agile Metrics for Predictability" and the original mind behind the ActionableAgile™️ Analytics Tool. Recently, he co-founded ProKanban.org, an inclusive community where everyone can learn about Professional Kanban, and he co-authored their Kanban Guide.
When he is not playing tennis in the Florida sunshine or whisky tasting in Scotland, Daniel can be found speaking on the international conference circuit, teaching classes, and creating amazing content for people like us.