At Lean Agile Scotland 2013 Chris Matts mentioned that “the WIP limits kill the options”. He didn’t really explain his statement in detail but I had been thinking about it and I dare to say that Chris was half right. He often talks about staff liquidity as well, which tells us how likely our staff will be able to work on a problem or a feature. In this context it means that “the more liquid our staff” the more work items we can work on in parallel, which means that we have more options. For example, we have four people in the team, two of them are expert testers, one of them is mediocre, and the last one has no idea about it. We can say that in this team three people can do testing. If this team has a WIP limit below 3, then this limit really kills options by preventing somebody who can do testing from actually doing it.
I mentioned that Chris was half right because if there is no limit on WIP the work might not get to the next phase. Imagine that the staff liquidity of the previously mentioned team for deployment is one (say it’s the member who has no idea about testing). If there is no limit on the testing phase the work can easily pile up there, which creates unnecessary pressure on the team member who does deployment, and the lead time will be unnecessarily long. So the right thing to do is to set the WIP limits based on the staff liquidity of the team and if you need more WIP in a phase, work on the staff liquidity instead. In real life it means that you train your people and help them grow. If you consider the team above, it is very risky to have only one team member who knows about deployment. Besides the risk of having a bottleneck (WIP limit 1 is a clear bottleneck) in your flow, this can negatively affect your lead time and throughput. Therefore, it is in your best interest to have a higher WIP limit (more options) in deployment, so either you help your current team members to learn more about deployment, or you hire someone who can do that.
These are equally good choices, and they are better than artificially increasing the WIP limit. Because if you think about it, if you have low staff liquidity then even if you have higher WIP in a particular column (more options - you can see how options and WIP are interchangeable) there won’t be more persons to do the job, so the work items may stay there idle and the lead time will not change: previously they were idle in the previous phase, now they are idle in this phase. Again, do not set the WIP limit. Use the characteristics of your system (e.g. the staff liquidity) to set them, and if you need higher or lower WIP, change your system accordingly, not your WIP.
Update: you can read Chris’ response here.
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