Toyota has this metaphor of “lowering the water level” that they use when looking for opportunities for improvement. This video explains the metaphor and how it applies to your Kanban system.
Imagine for a moment, a deep river with lots of rocks under the surface. The water moves quickly and smoothly downstream despite the rocks and the surface of the river is fairly smooth.
Now imagine that we can lower the water level to just above the rocks. Even though we can’t see the rocks themselves, we can see the effect they’re having on the water. We begin to see water being pushed up and over rocks. We see whirlpools and eddies forming in the surface. The rocks aren’t visible but their effect is obvious.
Now we imagine the water level dropping even further, exposing the rocks to the air. Now the effects of the rocks are obvious. We can see the water striking up against them and being pushed back.
The rocks never changed - only our perception of them did. When the water level was high, we could fool ourselves into thinking that the water was all moving quickly and smoothly because it appeared to be that way. We didn’t see the effect that the rocks was having on that water, under the surface. We didn’t see how they were interfering with the flow of the water.
We have the same effect in the work we do. When we have many items in progress (WIP), moving across the board, we can easily convince ourselves that the obstacles we have aren’t making much of a difference because lots of items still are getting completed.
Now when we reduce that WIP, we start to see the effects on the work. The fewer items in progress, the easier it is to see the obstacles that we have. Once we’ve removed those obstacles then the work will flow even better.
When we can’t see any more obstacles, lower the water level again.
Will there ever come a time that lowering the water level too much will impact our ability to get water to the destination? Absolutely yes, but I can almost guarantee that this isn’t today’s problem and likely won’t be a problem for quite a while.
If you’re not sure what to do to improve the system, lower the amount of work in progress.