While many Kanban practitioners worry about how to manage the variability in the work items, the greater source of variability is to be found in the people doing the work.
Here comes the Staff Liquidity concept where we assign the work not to the best people but to the average people that could barely do it.
The most versatile and capable people are left to float around and should not be allocated to any other tasks. So if a problem comes up, they are immediately available to address it. At the same time since they are not committed to a work item no work item will be late.
From TOC perspective using Staff Liquidity we build a capacity buffer ready to cover the special cause variations in the work process.
That in fact means we follow the Five Focusing Steps advocated by TOC:
1) We have identified the system constraint - the average people that could barely do the work. They are able to absorb the common cause variation in the work items.
2) We decided how to exploit the system constraint - we focus on keeping the constraint as busy as possible by giving the average people work they could barely do.
3) We subordinate everything else to the constraint - the most capable people are not the ones doing the work.
4) We elevate the system constraint - by training the average people that could barely do the work. At the same time we train the best people as well in order to ensure that the system constraint (in this case the average people) will not move.
5) We return to Step 1 and don't let inertia create a new system constraint.