Wednesday, December 02, 2020

Displaying Total Slack (Float), Free Slack (Float) and Negative Slack (Float) Graphically in MS Project 2019


Takeaway: Many confuse Free Slack, which can be enabled with a checkbox in MS Project, with that of Total Slack. Free Slack and Total Slack are completely two different concepts in management.  A number of books, guides inform the default slack available in MS Project to be Total Slack, which is incorrect. This post will outline how to enable the Total Slack, Free Slack and Negative Total Slack in MS Project. Slack is also known as Float in project management.

Free slack (or free float) is the amount of time you can delay a task, without impacting the early start (ES) of the successor(s). Consumption of free slack has no impact at all on the project’s finish date and if you have a number of tasks-say thousands-free slack is not going to be your top priority.

On the other hand, total slack (or total float) is the amount of time you can delay a task, without impacting the project’s end date or violating a schedule constraint. This is very important for management practitioners because, this slack will push your project’s finish date. 

To understand early start, and related schedule fields, along with the Total Slack or Total Float values, you can use this webinar. I’ve spoken at length about them.

Recorded Global Webinar: Two Pass Technique with MS Project


Enabling Slack (Free Slack)

Now, in MS Project 2019 software, there is an option to enable the free slack (NOT total slack), with a flip of a checkbox. When you enable the checkbox, the free slack/float value is shown. To enable the checkbox, go to: 

Format tab – Bar Styles – Slack checkbox. 

After you have enabled for slack, the free slack is displayed as shown below.

As shown above, we have 4 tasks:

  • Task A – 3 days duration, FF = 1
  • Task B – 5 days duration, FF = 0
  • Task C – 4 days duration, FF = 0
  • Task D – 2 days duration, FF = 3

Many wrongly take it for Total Slack/Float, and assume this is the time which must be protected in order to protect the end date of the project. 

The most important point to note here is that it’s for Free Slack, not for the Total Slack. A number of books, tutorials, guides inform this the slack, which when exhausted, will have impact on the critical path. This is not correct.

This can be further confirmed by checking the Bar Styles by going to:

Format tab – Bar Styles – Format – Bar Styles. 

As shown above the colored line shown to the right of the task is for Free Slack, NOT for Total Slack. 

Enabling Total Slack

For total slack, there is no checkbox available in MS Project 2019, which can be enabled or disabled to check the value in the graphical side of the Gantt Chart. 

You have to create the conditions for it in the bar styles to show. The condition created is shown below. I’ve added another one for “Total Slack” below the “Slack” field in Bar Styles and it’s from: Task Finish to “Total Slack”. The color for Total Slack is shown as Green

As shown above, I’ve added another condition for Total Slack, just below the condition for Slack, which is for Free Slack. The conditions given for the tasks are minimal, i.e., only Active ones are considered. You can add more conditions in the bar styles. 


As shown above, the green lines are for the Total Slack in the Gantt Chart. 

Enabling Negative Total Slack

The total slack can be negative in a number of situations. To understand when negative total slack/float comes, refer to this article:

Article: Negative Total Float with MS Project

For the project in this post, I would be adding a MFO constraint on the Finish Milestone to have the Negative Total Slack. This is shown below.

As shown above, I’ve applied the MFO constraint on Finish Milestone which resulted in a number of negative total slack for other tasks in the project.

Next, you can enable negative slack, again by making changes to the bar styles. This time, I’ve added another condition for the negative slack. Again, do note that it’s for the total slack, not free slack. 

The conditions given for the tasks are minimal, i.e., only Active ones are considered. You can add more conditions in the bar styles. 

Next, as you apply these bar styles to the Gantt Chart view, it will come as shown below.

In the above figure, the negative total slack values are shown in red color coding, whereas the positive total slack is shown in green color coding. 

Conclusion

This post addresses one of the biggest misconceptions for managers, planners and schedulers who use the MS Project software tool: Slack available as a checkbox in MS Project is for the Total Slack. It’s NOT. It’s for Free Slack. 

Don't think the consumption of this slack is a worrying thing for your schedule or for that matter protection of this slack will anyway help you to protect your end date. I’ve seen this misconception heavily used in various literature of MS Project.

I hope with this post, you get a better understanding on total slack, free slack, and how they can be visualized with MS Project. 


References:

[1] MS Project Live Lessons–Guaranteed Learning or Your Money Back, Satya Narayan Dash

[2] Global Webinar – Two Pass Technique with MS Project, conducted by Microsoft Project User Group (MPUG), presented by Satya Narayan Dash

 

 

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