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Float, Slack, Buffer

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Float (slack or buffer) is extra time that a task could consume beyond its duration estimate without impacting other aspects of the project. Total float is extra time without impacting the end date of the project and free float is extra time without impacting another project task.

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Float, Slack, Buffer

When to use

Float (slack or buffer) is a valuable project management resource. Some tasks have it and some do not. Knowing which is which helps to make day-to-day prioritization and risk management decisions. Float is calculated as part of calculating the critical path. If the critical path has not been calculated, you cannot know with certainty how much float you have or which tasks have float.

On schedule-driven projects, I calculate the critical path and determine where there is float available. I preposition float as a risk mitigation response, allocating a portion of float into the schedule immediately following high-risk tasks. There is no float available for critical path tasks if a project has been scheduled to complete as soon as possible. (By definition, the critical path is the path with no float.)

Instructions

To calculate float (slack or buffer) first calculate the critical path. Subtract the “Early Finish” value from the “Late Finish” value. If the values are the same, the float is zero and the task is on the critical path. If the value is negative, an error has been made somewhere, recheck your calculations.

Float is created through the project planning process. Based upon the network diagram for the task schedule and the task duration estimates, float can be calculated. The amount of float available to a task can be changed by changing the network diagram or changing the duration estimates of that task or other tasks in the project. Float management is an excellent form of risk management on schedule-driven projects.

Float is a very valuable resource for resolving issues. When an issue arises, resources can be “borrowed” from tasks with float to work on solving the issue. As long as those resources are returned to their original task before all the float is consumed, the project will not be delayed.

Float can be categorized as Free Float or Total Float. The calculation referenced above calculates Total Float. This is the float available to a task before a delay would impact the end date of the project. The best way to calculate Free Float is to subtract the task Total Float from the successor task Total Float. Free Float is the amount of float available to a task before it begins to impact any other task, not just critical path tasks.

Definitions

Total Float: "The amount of time that a scheduled activity can be delayed or extended from its early start date without delaying the project finish date or violating a schedule constraint." PMBOK^® Guide

Free Float: "The amount of time that a schedule activity can be delayed without delaying the early start date of any successor or violating a schedule constraint." PMBOK^® Guide

This definition is taken from the Glossary of the Project Management Institute, A Guide to the Project Management Body of Knowledge, (PMBOK^® Guide) – Sixth Edition, Project Management Institute, Inc., 2017.

00:04 Hi, I'm Ray Sheen.
00:05 I'd like to talk about float, slack, buffer.
00:08 Those terms are used interchangeably, but
00:10 they're an important concept in schedule planning.
00:13 To understand the concept of float, slack, or buffer,
00:16 let's work with the metaphor of a window.
00:19 Every task has a time window in which we need to accomplish the task.
00:22 The task cannot be started until the window opens, and the task must be
00:27 finished before the window closes for the project to remain on schedule.
00:31 As long as the task is accomplished when the window is open,
00:35 the project will not be delayed.
00:36 Sometimes, the window is open much longer than is needed to complete the task.
00:41 You might have six weeks to do a five-day task.
00:45 Sometimes, though, the window is open for
00:48 exactly the same length of time as the task.
00:50 I that case you, may only have five days to do a five-day task, so
00:54 you need to start that task the minute the window opens so
00:57 that you'll be able to complete it when the window closes.
01:01 I know you're thinking that sometimes the time window is smaller than the duration
01:05 of the task.
01:06 No, it's not.
01:07 If you think that's the case during project planning,
01:10 you need to change the plan.
01:11 Don't start a project that you know will fail.
01:14 If you don't have enough time, you should see the stakeholders to either get
01:19 the time boundary extended, the scope reduced, or more resources added so
01:24 you can complete the work within the allowable time window.
01:27 Extra time within a time window is referred to as float, slack, or
01:32 sometimes it's called buffer.
01:34 I don't care which term you use,
01:36 I will be using float in the remainder of this lesson.
01:39 Be aware that the term buffer is used with a different definition when doing
01:44 critical chain project scheduling.
01:47 So, why do we care?
01:49 How do we use float to help manage the project?
01:52 Float is created based upon the schedule plan that you establish for the project.
01:57 If you have multiple paths of project activities,
02:00 you're almost guaranteed to have float.
02:03 The predecessor-successor relationships between tasks and the estimate of the task
02:07 duration will determine how long each of those paths will take.
02:11 The longest path is the critical path, and has no float.
02:14 All the other paths have float.
02:18 Changing the product schedule plan will change the float,
02:22 either changing the relationship or the estimate.
02:25 If the duration of one of the paths change, the float will change.
02:29 I use my float as a schedule reserve.
02:31 I want to know which paths have float and how much they have, so
02:35 that I know which paths have schedule reserve.
02:38 Paths with no float by definition are critical paths.
02:43 However, normally, the majority of tasks are not on the critical path and
02:48 they have some float to work with.
02:50 So, what do I do with that reserve?
02:51 I normally use it in one of three ways.
02:54 One approach is to position float immediately following a high risk task.
02:58 It may be high risk because of estimate uncertainty.
03:03 It may be high risk because of technical difficulty.
03:06 It may be high risk because of resource constraints.
03:09 Whatever the reason, the float is there to accommodate
03:13 an unplanned delay in that task without an impact on any other task.
03:18 I will also position some of the float right before a major milestone is due.
03:22 Many times,
03:23 when I approach a milestone there are little last minute problems that surface.
03:27 The float time is there to resolve those without impacting the milestone.
03:32 Finally, I use float to resolve resource over allocation problems.
03:37 If you have a resource over allocated at a particular time in the project schedule,
03:42 I try to use the float to move one of the active tasks at that time to a later time
03:47 within that task's time window.
03:49 There are two ways to calculate float, total float and free float.
03:54 Let's compare the two.
03:55 The Project Management Body of Knowledge, the PMBOK Guide, defines
03:59 total float as the amount of time that a scheduled activity can be delayed or
04:03 extended from its early start date without delaying the project finish date or
04:08 violating a schedule constraint.
04:10 This is calculated for us with a critical path calculation, and
04:14 it's the latest finish time minus the earliest finish time for each task.
04:18 Of course, when those times are equal, the answer is 0, and
04:21 that tells us that task is on the critical path.
04:24 But as long as we have a positive number, there is task total float.
04:28 The impact of exceeding the value of task total float is to
04:32 cause a delay to the end date of the project.
04:36 Using up only a portion of the float will likely impact successor tasks,
04:40 although it will not impact the end of the project.
04:44 So let's compare task total float with the other type of float, task free float.
04:49 The Project Management Body of Knowledge, the PMBOK Guide, defines free float as
04:54 the amount of time the schedule activity can be delayed without delaying the early
04:58 start date of any successor or violating a schedule constraint.
05:02 That means it's the amount of float that the task can use without impacting
05:06 anything else.
05:07 It's calculated by starting with the total float for a task and
05:11 subtracting the successor task total float value.
05:14 If both tasks are on the same non-critical path with no other
05:18 relationships intervening, they will normally have the same total float
05:23 value.The free float value is then 0.
05:26 A delay of the first task impacts the start of the second.
05:29 The impact of exceeding task free float is on the next task or event in the project.
05:36 Using up only a portion of the task free float will not impact the schedule
05:40 of anything else in the project.
05:42 If you're planning on taking the PMP exam, be certain that you understand
05:47 the difference between these two, and how to calculate them.
05:50 The use of float is an excellent way to manage schedule risk on your project.
05:57 If your project has schedule constraints, it's imperative that
06:02 you understand which tasks and paths have float and which ones don't.
06:08 This is your schedule reserve.

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Float, Slack, Buffer

About this lesson

Exercise files

Quick reference

Float, Slack, Buffer

When to use

Instructions

Definitions