Column WIP limits…
… If any improvements develop, then those improvements occur by accident and happenstance. They originate from those interventions that just happen to affect Herbie, the real constraint. The fact that they involve the real constraint is merely coincidental, as a lucky combination of column WIP limits that happen to expose the real constraint, in that particular circumstance.
Unfortunately, this effectively reinforces the conviction that the approach is valid; because it does produce tangible and positive improvements after all.
Too bad though that the majority of the interventions are completely ineffective. They are a waste.
It is much wiser to make a deliberate effort and focus interventions where they are needed. On the real constraint, on Herbie.
This effect might escape attention precisely because positive results and improvements over any other earlier approach will indeed ensue; but these results and improvements come from the open discussion that are provoked about problems, and the coincidental fact that, at times, they truly affect the real constraint.
Learn to focus on the real constraints and make all your improvement efforts efficient and effective.
Quoted from: Tame the Flow.