Slack Is for Optional Stuff, Not for Important Stuff

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.

We call that slack time.

Slack Time

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.

Important 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.

Optional Stuff

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.

(Photo by Smaedli, Creative Commons)

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  • PiotrUryga

    Good point and I agree on most part, but not on Parkinson’s Law .
    It does not apply to software development teams.
    You’re mentioning DeMarco’s book, so take a look at another one: “Peopleware”. It has whole chapter on why Parkinson’s Law is completely irrelevant to team’s developing software.

  • Glen B Alleman

    Slack – schedule margin – protects your deliverable from being late, like missing a flight. This slack time should not be lost, but with work moved to the left, but the work has to be ready to be moved.
    In agile construction this is called “make ready,” work is ready to go if the schedule margin is not used.
    One missing notion in most agile plans we see is there is no schedule margin, only reduced stories. In the more formal scheduling domains, this would not pass the smell test. What this means is the team can’t assure the stakeholders they will show up on time with the planned features, since variance in all things is “normal.” Without schedule, cost, and technical performance margin, you’ll be late, over budget, and there will be problems.

  • Yves Hanoulle

    Thank you for this article.
    You are so right that people make the mistake of counting on slack time to do extra things.
    It’s why people don’t have time to read books or don’t have any money left at the end of the month.
    If it’s important you have to plan it.
    That said, when I have time left, I still use it for important things. I will read some extra pages in some business books, or write a few pages of my own book. (Or at the end of the month set some extra money away)
    But these are extra’s. On top of the normal nr of pages I read every day. (Or normal amount of money I move to my savings account at the beginning of the month)
    When I do this, it’s very tempting to then start counting on the slack time. And then the next step is to stop planning for the important things. And then I am in trouble.

  • Marcin Floryan

    It might be just semantics but I see your example slightly differently. What you describe as the 1.5 hours that you add to get to the airport I would not call “slack” I would call this “contingency”. This is your risk mitigation. You add extra time to make sure, that if something unexpected happens, you still make it on time. Although I’m sure organisations (particularly traditional organisations) don’t do enough of it in I.T. every single classic PM approach (I bet PMBOK is full of this) tells you to add sufficient contingency to make sure we meet a deadline. And even if we do add contingency (Hofstadter’s Law: It always takes longer than you expect, even when you take into account Hofstadter’s Law) 1.) Things we didn’t expect happen and we use it up (it was needed after all) and 2.) We tend to use it up anyway for activities that have very little value.
    Now as to Slack, going with what I understood from De Marco and totally agreeing with the utilisation problem you talk about, I understand it more as organising the work in such a way that people are not busy 100% of the time (for example, by introducing WIP limits) rather than explicitly adding time since, as Yves rightly points out, adding extra time often means we use it up (thus keeping ourselves busy).

  • Jurgen Appelo

    Slack time, contingency planning, 80% utilization, it’s all (roughly) the same thing…

  • Glen B. Alleman

    Slack – or float or total float – is the schedule or cost margin planned to be used to protect the deliverable.
    “amount of time that spans from the completion of one previously scheduled activity and extends to the point at which the next scheduled activity is set to begin.” PMBOK 4th edition
    – Jurgen has a extra 30 minutes before his flight to read a book.
    Contingency is planned cost or schedule held back to cover in scope but unplanned work
    “The term contingency reserve refers primarily to the amount of quantity of funds or other financial resources that is required to be allocated at and above the previously designated estimate amount to reduce the risk of overruns to an acceptable level for the financially responsible organization.”
    – Jurgen has $10K and 1,000 hours in contingency reserve to cover in scope but unplanned work on the project.
    Utilization is the absorption rate of those working on the project. Jurgen is currently consuming 30 of the 40 hours a week planned for his assignments and is running at a 75% utilization.
    These three terms are separate, distinct, and well defined in the project management domain.

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