Value flows quickly through a lean factory all of the time. This high throughput is sustained because faults in the product and process are continuously removed. But in order to achieve such continuous process maintenance, the plant must be over-staffed. A lean factory employs more than the bare minimum number of people required to support current flow rates. Because the preservation of tomorrow’s flow rates depends on eliminating systematic problems. And in a constantly changing world that requires effort.
I read somewhere that Toyota over-staffs by about 15%. That is, workers spend about 85% of their time adding value, and the other 15% ensuring that the current pace is sustainable. Note that value is being added 100% of the time, and product is flowing at the rate demanded by the market. It’s just that by adding one extra person in every six, there’s now a little slack in each team. So when Murphy strikes, the team’s process can be fixed without diverting effort from adding value.
Some Western organisations have taken this 15% slack time and cordoned it off. “Everyone spend 15% of your time improving our processes,” they say. This is undirected effort. Better to have that slack available, and to fix real problems as they occur. This contributes more directly to protecting the value stream.
So an agile software development team should only commit to delivering 85% of what it knows is possible each iteration. The remaining 15% should be used for kaizen activities, and for dealing with quality incidents as they occur. Without that slack, bad luck will either impact today’s schedule or tomorrow’s quality. Either way throughput will be compromised, and the project is on the road to failure.