Why must efficient systems have slack?

Tom Breur

8 September 2013

Efficiency. According to an online dictionary it’s “the ability to accomplish a job with a minimum expenditure of time and effort” or “the ratio of work done to the energy supplied to it.”

Since Frederick Winslow Taylor we have been indoctrinated by the Cost Accounting paradigm. For the industrial revolution, and through the first half of the 20^th century, it worked so well.

That is why we (still) think of buzzing factories and busy people, all frantically running around, to keep all machines running at full speed. But efficiency in knowledge work requires a different view, even a different paradigm.

An efficient system must have slack. In knowledge work, creating products in teams, you need some “leeway” to swarm around bottlenecks that may be rapidly changing location. Everybody must be on their feet to spot the bottleneck (or CCR, Capacity Constrained Resource). Hard to do, if you’re busy 24*7.

You need (some) spare capacity to swarm around the identified bottleneck. Which also shows the importance of cross training, so that people can chip in for each other. When do you prepare that? So-called slack is great for ad hoc on-the-job self monitored training.

Fully loading a team is a little bit like a highway with cars backed up. They cannot move very fast. If at all. Or like shelves that are neatly organized, but completely stacked. It becomes very difficult to keep them neat. Just because every little slot is taken, only an extremely neat organization will allow you to find your stuff.

In tightly organized shelf space, it takes only a few items shifting around, and you quickly need to move several piles to get to your stuff. Empty shelf space to help you keep them organized is like slack in a system: it prevents slight disruptions from rippling through it. Otherwise it becomes a house of cards.

The Theory of Constraints carries several (initially!) counter intuitive implications like these. Every resource in a production chain that is not the bottleneck, is effectively producing excess Work-in-Progress, which leads to excess inventory. Inventory eats up cash, and of al the things that can happen to inventory, only one is good: getting sold. The rest is waste…