Column WIP limits…

… bring about system instability. This induced instability effectively disrupts the applicability of Little’s Law.

It is ironic that limiting work in process is justified by the need of getting to a stable state, so that Little’s Law applies; but column WIP limits break this condition, rather than promoting it.

Furthermore, when the system is in an unstable state, most variability will come from special cause variation. Thus any opportunity to identify — let alone improve off — common cause variation will be lost. A system is stable when it is affected only (or mostly) by common cause variation. Therefore, if one is able to keep the system in a stable state, you will have two significant advantages: you can apply Little’s Law and you get the opportunity to identify common cause variation.

Implementation of the Kanban Method with column WIP limits forgoes entirely the vast improvement opportunities that can be found in dealing with common cause variation.

There are better ways! Read about them in: Tame the Flow.