Float & Slack
Float & Slack? What are they? and what is the difference between ‘Free Float’ and ‘Total Float’?
As a very quick answer Float = Slack – they are the same thing!. All of the European and Middle Eastern training delegates I have met are familiar with the term ‘Float’, but MS Project has been created by an American organisation, and Americans (and MS Project) refer to ‘Slack’.
So what is Float & Slack?
Float & Slack. Float is the difference between the ‘earliest starting time – EST’ and the ‘latest starting time – LST’ of a task. For a critical task, the EST and the LST will be the same and there will be no (zero) float. For a non critical tasks the LST will be greater than the EST, and hence the task can be delayed until its LST.
Float
In the above diagram all of the tasks are linked by dependencies. Task 2 is not on the critical path, and can be delayed by a few days. This is because Task 3, which starts at the same time as Task 2, takes longer to complete.
I have displayed the ‘Critical Path‘ and ‘Slack’ by checking the boxes on the Format menu, although this only shows a thin black line for slack.
Slack Check Box
My preference is to use the ‘Format – Bar Styles’ option to customise slack to be more visible. In the image below I have edited both the colour and the bar shape to make the Slack/Float more visible.
Bar Styles
Resulting in a more visible display of Float in the image.
Green Float
Why is showing Slack Important?
Well the Critical Path shows tasks that if delayed will delay the whole project. But of near equal importance are the tasks that are ‘Nearly Critical’ – those that with just a day or 2 delay could change the critical path.
MS Project gives us an option for showing these as critical tasks in the ‘File – Options – Advanced’ Menu – right at the bottom. I have changed this from ‘0’ to ‘3’.
Nearly Critical
Resulting in a task that is marked as critical, but that also has float!
Tasks with Float, but Critical!
Float can also be seen by adding a column into the table area in MS Project (You could then ‘Sort’ by this column to find tasks with limited ‘Float’)
Start Slack Column
Now that the basics are understood, let’s consider a slightly more detailed project. Here I have inserted a new task (Task 2a) after Task 2. Task 2 and 2a are not critical, because they take less time than Task 3.
Total Slack
But note the Slack column, and the image. Task 2 is now shown with no Green Slack in the image, but is shown with 1 day slack in the table. This is because Task 2 can be delayed by 1 day – but only by “taking” the slack from Task 2a. What we are actually showing here is ‘Total Float’ or ‘Total Slack’ – the ‘Slack’ available before delaying the Critical Path.
‘Free Float’ or ‘Free Slack’ is the ‘Slack’ available before delaying any of the next tasks. We can insert a column onto the table area called ‘Free Slack’ to show this.
Free Slack
We can set up a custom Bar Style to show Free Slack.
Free Slack Bar Styles
Note: I have edited the order of these Bar Formats as they are processed in order from top to bottom.
Free Slack Gantt Chart
Float and Slack are the same thing! ‘Total Float’ the default setting on MS Project shows the time available before delaying the critical path. ‘Free Float’ shows the time available before delaying any following tasks.
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