Total Slack and Free Slack for Project Decisions

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Working with seasoned project managers, schedulers, and countless students in my Microsoft Project classes, I see a lot of people focusing heavily on the critical path and how the tasks on it drive business decisions. However, it is also important to know how to evaluate what is not on the critical path and how to take a look at available slack time. This knowledge can help you figure out when following the critical path might be at risk; in some cases, before the project has even started.

## Show Critical Gantt Bars

In Project 2010, to display the critical path go to the **Format** tab, click the **Critical Tasks** checkbox.

Notice that throughout the schedule, critical tasks are marked with red Gantt bars and non-critical tasks remain blue.

## Show Free and Total Slack Fields

In Project 2010, to display additional fields do the following:

1. On the **Format** tab, in the Columns group, click the **Insert Column** button and select **Total Slack**.

2. Repeat and select **Free Slack**.

3. Drag the columns to their desired location.

Notice that all critical tasks for this schedule display 0 in the Total Slack column.

This follows the traditional definition of a Critical Task that you may have seen from PMI or Advisicon materials – when the earliest date a task can start is equal to the latest date a task can start, the task is considered critical because its available delay time is zero days (i.e., 0 days of slack time). Critical tasks must stay on schedule to honor the project end date.

Critical Tasks with Zero Total Slack

Critical Tasks with Zero Total Slack

## What Free Slack Tells Us

Notice that in the Tradeshow Promotion portion of the schedule all of the tasks have Total Slack, but a few also have free slack.

Illustration of Total Slack and Free Slack Variances

Illustration of Total Slack and Free Slack Variances

If the only column you pay attention to is total slack and you delay some of these tasks to free up resources for critical tasks, you may not realize that other non-critical tasks in the schedule will also shift.

The advantage of Free Slack is this informs you of the number of days of slack time that are available to you where nothing else in the schedule will shift; such as a successor task. Best practice when delaying tasks to correct for a situation (e.g., resource shortage) in your schedule is to delay the less risky tasks which have free slack.

## What Negative Total Slack Tells Us

A very valuable situation to project managers is when Total Slack displays a value that is less than zero (negative).

Negative Total Slack on a Critical Task

Negative Total Slack on a Critical Task

This seems like an impossible situation since critical tasks are at zero, you may be wondering what a negative value means. In Microsoft Project, a negative Total Slack is an indicator of a missed deadline. If that task – or even the entire project – has not started, this schedule is already in trouble. This is an excellent time to work with the resource manager, sponsor, team or others to get the schedule back on track to avoid the missed deadline.

## Some Takeaways

* Next time you are evaluating your schedule and making a decision about which non-critical tasks to delay in order to free up a resource, consider choosing the tasks with Free Slack first and delay within the Free Slack value to keep the rest of the schedule the same.

* If you use deadlines, display the Critical Path with Total Slack to discover when tasks are already scheduled to be late and make adjustments before you reach that point in the schedule.

I hope you found this information useful. For more information on scheduling and Advisicon best practices, please sign up for an upcoming course in [Managing Projects with Microsoft Project 2010](http://managing-projects-with-microsoft-project-2010-cl.eventbrite.com “Register for the public “Managing Projects with Microsoft Project 2010” course”).

*[PMI]: Project Management Institute

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Showing 5 comments
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    David Rowden

    Hi
    I have been reading some of your articles and wonder if there is aan easy solution to my problem, which you may be able to answer really quickly. I want to show Free Slack on my bar chart in MS Project, not Total Slack, can this be done.

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    LewisCindy

    Hi David, Have you discovered the feature on the Format tab for Slack (checkbox)? This displays a Slack line on the Gantt chart. If you then review the details on the Format tab, Format dropdown, Bar Styles, you will see the configuration. It is already configured for Free Slack. The configuration can be changed easily in Bar Styles or you can add a second skinny bar to show Total Slack. Hope that helps. Cindy

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    Deepak Gupta

    Hi, i have a query, what does it mean if slack & total slack time are both same for a task in project schedule.
    Regards,
    Deepak

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    Nikhil

    Hi,
    can free slack time be a negative value?

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    Alex Zamani

    Hi,

    I have a question about Slack,
    I am preparing a construction schedule for my small project and using M.S.P. the problem I have is, when I active the slack on the schedule, I realize that on the total slack column, I can not see the total slack for whole project and shows ZERO but on the individual items shows we have slack. How come I can not see total slack on my whole project??? even if I add some more tasks on the critical items again on the top of my schedule ( usually it is name of a project and you can see duration of project, total slack for whole project) show total slack for whole project is again ZERO. Could you please explain me, who can I show on my schedule how many days we have + or – slack for whole project?

    Thank You,
    Alex Zamani

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