One of things you can hear repeatedly from me is why we should limit work in progress (WIP) and how it drives continuous improvement. What’s more, I usually advise using rather aggressive WIP limits. The point is that you should generate enough slack time to create space and incentive for improvements.

In other words, the goal is to make people not doing project or product development work quite frequently. Only then, freed from being busy with regular stuff, they can improve the system which they’re part of.

The part which I was paying little attention to was the cost of introducing slack time. After all, these are very rare occasions when clients pay us for improvement work, so this is some sort of investment that doesn’t come for free.

This is why Don Reinertsen’s sessions during Lean Kanban Europe Tour felt, at first, so unaligned with my experience. Don advises to start with WIP limits twice as big as average WIP in the system. This means you barely generate any slack at all. What the heck?

Let’s start with a handful of numbers. Don Reinertsen points that WIP limit which is twice as big as average WIP, when compared to no WIP limit at all, ends up with only 1% idle time more and only 1% rejected work. As a result we get 28% improvement in average cycle time. Quite an impressive change for a very small price. Unfortunately, down the road, we pay more and more for further improvements in cycle time, thus the question: how far should we drive WIP limits?

The further we go the more frequently we have idle time, thus we waste more money. Or do we? Actually, we are doing it on purpose. Introducing slack to the system creates an opportunity to improve. It’s not really idle time.

Instead of comparing value of project or product work to idle time we should compare it to value of improvement work. The price we pay isn’t that high as it would initially seem basing simply on queuing theory.

Well, almost. If we look at the situation within strict borders of a single project value of improvement work is non-existent or intangible at best. How much better will the product be or how much faster will we build remaining features? You don’t know. So you can’t say how much value these improvements will add to the project.

However, saying that the improvements are of no value would be looking from a perspective of optimizing a part; in this case a single project. Often impact of such improvements will be broader than within borders of the project and it will last longer than the project’s time span.

I don’t say I have a method you may use to evaluate cost and value attached to non-project work. If I had I’d probably be a published author by now, had lots of grey hair and a beer belly twice as big. My point is that you definitely shouldn’t account all non-project work as waste. Actually, most of the time cost of this work will be smaller than value you get out of it.

If we based purely on Don Reinertsen’s data and assumed that whenever we hit WIP limit people are idle we could come up with such a chart:

On a horizontal axis we have WIP limits going from infinite (no WIP limit at all) to aggressive WIP limits inflicting much slack time. On a vertical axis we have overall impact on a system. As we introduce WIP limits (we go to the right side of the chart) we gain value thanks to shorter average cycle times and, at least at the beginning, improved throughput. At the same time we pay the cost of delay of rejected or queued work waiting to enter the system (in backlog) and the cost of idle time.

In this case we reach the peak point of the curve pretty quickly, which means that we get most value with rather loose WIP limits. We don’t want to introduce too much idle time to the system as it’s our liability.

However, if we start thinking in terms of slack time, not idle time, and assume that we are able to produce enough value during slack time to compensate the cost the chart will be much different.

In the case number two the only factor working against us is cost of delay of work we can’t start because of WIP limits. The organization still has to pay for people doing non-project work, but we base on assumption that they create equal value during slack time.

The peak of the curve is further to the right, which means that the best possible impact happens when we use more aggressive WIP limits than in the first case.

Personally, I’d go even further. Basing on my past experience I’d speculate that often slack time results in improvements that have positive overall impact on the organization. In other words it would be quite a good idea to fund them as projects as they simply bring or save money. It gives us another scenario.

In this case impact of slack time is positive so it partially compensates increasing cost of delay, as we block more items to enter the system. Eventually, of course, overall impact is negative in each case as at the end of horizontal axis we’d have WIP limit of 0, which would mean infinite cost of delay.

Anyway, the more interesting point to look at is the peak of each curve, as this is a sweet spot for our WIP limits. And this is something we should be looking for.

I guess, by this time you’ve already noticed that there are no numbers on the charts. Obviously, there can’t be any. Specific WIP limits would depend on a number of context-dependent factors, like a team size, process complexity or external dependencies, to mention only the most obvious ones.

The shape of curves will depend on the context as well. Depending on the work you do cost of delay can have different impact, same as value of improvements will differ. Not to mention that cost attached to slack time vary as well.

What I’m trying to show here is that introducing WIP limits isn’t just a simple equation. It’s not without a reason that no credible person would simply give you a number as an answer for a question about WIP limits. You just have to find out by yourself.

By the way, the whole background I drew here is also an answer for the question why my experience seemed so unaligned with ideas shared by Don Reinertsen. I just usually see quite a lot value gained thanks to wise use of slack time. And slack time, by all means, should be accounted differently than idle time.