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15 January 2011

What is Slack? What is Float? Is There A Difference?

David A. Zimmer
David A. Zimmer, PMP
Chief Project Professor
American Eagle Group
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PMBOK® Definition:

Slack: Also called Float. See Float.
Float: Also called Slack. See Slack.

Total Float: The total amount of time that a schedule activity may be delayed from its early start without delaying the project finish date, or violating a schedule constraint. Calculated using the critical path method technique and determining the difference between the early finish dates and late finish dates. See also Free Float.

Free Float: The amount of time that a schedule activity can be delayed without delaying the early start date of any immediately following schedule activities. See also Total Float.

Practical Definition:

Slack or Float provide flexibility in the project schedule. When leveraged properly, project managers can shift activities and resources to meet the project objectives and priorities. It is the amount of time an activity can be delayed without impacting other activities or the project end date and changes over the course of the project implementation.

So, What Is Slack or Float?

Slack and float are the same thing. They can be used interchangeably, so our discussion of slack and float will purposely use both. Once the concept is understood, it is more important to understand how to leverage slack time within projects.

We discussed float in the definition of the Critical Path because the critical path has no slack or float by definition. The non-critical path(s) contains the float – the amount of time an activity can be delayed without delaying successor tasks or the project end date.

A Bit o’ History

Modern Project Management practices were developed starting in the 1950s. Much of the credit goes to the US Navy while developing the Polaris project: a missile launch system from a submerged submarine. Prior to that period, no formal methodology was used to manage projects. Because of time pressure, highly classified and cutting edge developments, the leaders felt it important to standardize the implementation of the project.

It developed the Project Evaluation and Review Technique, a.k.a., PERT for managing the projects. From that methodology, we have the Work Breakdown Structure, Work Packets, PERT Analysis, and more. They coined the term Slack (float has a different meaning for the US Navy).

Meanwhile, private industry was coming to a similar conclusion: the lack of project management methodology caused many projects to fail or not be completed according to plan. It developed the Critical Path Method which determined the path of activities that defined the project’s shortest duration. From it, we gained the concept of the Critical Path, Critical Tasks, Non-critical Path, Non-critical Task, etc. They used the term Float.

Today, we follow the Project Management Body of Knowledge developed by the Project Management Institute which is a compilation of the two methodologies keeping the best of both.

Calculating Slack or Float

Total Float

In our critical path definition article, we described the method of forward passes and back passed to determine the critical path of the project network diagram, the early start and finish of each activity and the late start and finishes. Using the early start (ES) and late start (LS) information generated while calculating the critical path, we use the following formula to determine Total Float – the amount of time a task can be delayed without delaying the project end date.

Total Float = LS – ES

Each task along the non-critical path will initially have the same Total Float value.
Assume the following notation represents a specific task where

  • ES = Early Start
  • EF = Early Finish
  • LS = Late Start
  • LF = Late Finish
  • DU = Duration

Using the same network diagram we used in the critical path definition, shown below, we see the Total Float (TF) equals eleven (11). Note, it is the same value for the entire non-critical path (the top path). As the project progresses, various tasks may slip, be delayed or take longer than originally planned. This change decreases Total Float.


For example, assume Activity C slips by 3 days. The remaining total float would be 8 days. In fact, since Activity C slipped by 3 days, so do Activities D and E. Their Total Float values decrease to 8 days, also. If Activity D takes 10 days instead of the planned 8, Total Float will decrease another 2 days. Activity E’s Total Float then becomes 6 days.

If Activity E subsequently slips for whatever reason more than 6 days, the project will be delayed. So you can see how Total Float changes over the length of the project. This is normal of projects.

Our diagram is simple. In typical project schedules, many non-critical paths exist, each with different values for their total float. As the project progresses, their float values will change at different rates as their respective activities delay.

Free Float

Let’s understand Free Float before we discuss how to leverage this knowledge.
Free Float is the amount of delay an activity can experience without delaying any of its successor tasks. Typically, the only place you’ll see Free Float is at the end of a non-critical branch, unless the team purposely placed buffers between tasks.

Buffers are also called Reserve in the PMBOK. The PMBOK defines Reserve as a provision in the project management plan to mitigate cost or schedule risk. The reserve we are discussing here is a schedule buffer. Buffers are placed in the schedule after high-risk tasks or tasks that have a high risk of slipping, such as “Waiting for material from an outside vendor.” These can experience shipping delays, manufacturing issues, vendor priority changes, etc. These conditions can have severe results on project schedules. We lower the risk’s probably or impact (mitigation) by placing buffers after those tasks.

Back to Free Float.

We use the following formula to calculate Free Float:

Free Float = ESs – EFp - Lag

We calculate Free Float by subtracting the EF of the predecessor from the ES of the successor minus any lag between the two. Let’s understand this conceptually. Any value greater than zero represents the amount of delay the predecessor can delay without impacting the successor. If it has multiple successors, the predecessor’s Free Float is the minimum value of all those calculated.

In other words, let’s say that once the predecessor finishes the successor can start (the classical Finish-to-Start relationship). If the successor can start without any delay after the predecessor finishes, than the Early Start (ES) of the successor equals the Early Finish (EF) of the predecessor, therefore, ESs – Efp equals zero, there is no Free Float. Even if a lag is defined between the two tasks, Free Float would be zero because lag does not change if the predecessor slips – it simply pushes the successor task ES by the same amount of the predecessor’s delay.

In our diagram, we see Free Float (FF) in the top path equals zero for all tasks except Activity E. That is true because once the predecessor task finishes, the successor task is scheduled to start immediately. Activity E has Free Float because its successor’s ES is determined by Activity G, which finishes later, therefore, providing Free Float for Activity E.

Observation For PMP Exam Takers

For those looking to take the PMP exam, note some of the following observations:

  1. TF and FF are zero for every task in the critical path. That follows the definition and, if calculated mathematically, would prove to be true. Thus, once the critical path is determined, you know those values will be zero.
  2. If, while conducting the back pass and all calculations are correct, you have a value greater than zero upon reaching the first task of the network diagram, that value represents the TF and FF for the non-critical path (of course, if there is only one non-critical path in the network diagram).
  3. FF only exists at the end of the non-critical path if there are no buffers placed in the path.
  4. Be careful to look for lag designations such as the one between Activity F and Activity G.

Practical Application

You are thinking, “While all this understanding is fine and dandy, what does it have to do with the real world and how can I leverage it? Oh, and by the way, I’m not going to be calculating this stuff, so why the big definition?”

Certainly, in a real project where 400 to 800 activities exist, you will not be calculating this information by hand. Instead, using project management software such as MS Project (MS Project 2010 shown below), you can see and watch, as well as leverage, the float time occurring naturally or see the buffers placed after high risk tasks.


In the Gantt chart shown, the time illustrated between the green lines represents Total Float. It shows the amount any one of the tasks in the Extruder portion of the project can be delayed without delaying the project end date. It also represents the Free Float between the Extruder Delivery and the end of the project. Notice there is no other Free Float on that path or on the Critical path (shown using red Gantt bars).

How to Leverage the Free Float and Total Float

As the project progresses, the “whitespace” will change, showing the impact on the float time. You’ll want to watch it so it doesn’t “disappear” causing the project or successor tasks to be delayed.

Let’s say one of the critical tasks (shown by red Gantt bars) is not progressing as planned. If adding additional resources to the task would help bring it back in line, then moving resources from non-critical tasks might make sense. We can see the impact of moving resources from one task to another because the software will update the plan accordingly – assuming the tasks are effort-driven and developed properly. As a resource is removed from the non-critical task, its duration will lengthen. By placing the resource on the critical task, its duration will shorten. We can see the impact on the float and determine if it is acceptable.

Because our plan is in some project management software, we can play several “what-if” scenarios to determine the best avenue of re-aligning resources before we actually make the changes permanently. In this manner, we can avoid some costly mistakes.

Conclusion

Slack and Float are the same thing. We differentiate between Total Float and Free Float to understand the amount of delay we can tolerate without impacting the project schedule and successor activities, respectively.

The concept and use of float helps us keep our project on schedule and use resources more efficiently as the project progresses. The lack of float or its disappearance gives us clear indication our project is out of control and corrective measures must be taken. Additionally, by using what-if scenarios, we can understand impacts to the project

Understanding float and its use is one industry trick of successful project managers.

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