When I plan to catch a flight I usually schedule up to 3 hours from the moment I leave my house to the moment the gate closes. I know I probably don’t need those 3 hours. I rarely do. If all goes well it costs me less than 1.5 hours, and I have at least 1.5 hours of extra time available at the airport.
Slack time is extra time that we build in as a safety mechanism. In case we need it. Hopefully we don’t need it, and then we put it to good use when it becomes available. Like shopping for Bowmore 17 Year Old, or reading Tom Demarco´s book at the gate, or tweeting from the KLM lounge.
The amount of time that a non-critical path activity can be delayed without delaying the project is referred to as “slack time”. – someone on StackOverflow
Many experts have suggested to schedule slack time in your projects. This is a Good Thing. Because the science says 100% utilization is not going to work. You should aim for just 80% or even 60% utilization, and use the slack for optional stuff.
Some people suggest to use slack time for improvement, refactoring, learning, and other important activities. Well, that can be a Good Thing. But not necessarily so.
You see, slack time is time that might become available. If all goes well.
But in software projects things rarely go well. Finishing work in projects is certainly not as easy as catching a flight. Which means that slack time is usually lost. (And Parkinson’s Law certainly contributes to this problem.)
And for many people, who use slack time for important stuff, no slack time means no improvement, no refactoring, and no learning.
Slack time is not meant to be used for important stuff. Important things, like continuous improvement, refactoring, and learning, should be scheduled separately. These activities should not depend on slack time becoming available.
I don’t brush my teeth in slack time. I do it always.
I read my novels in slack time. I do it only when a day went well.
For me, writing is also an important activity. I schedule my writing time separately, to make sure I have time for it. Sure enough, sometimes I have extra time to write in the KLM lounge. That´s nice. Once I even brushed my teeth there. But I never count on it.