Float, Slack, Buffer

Quick reference

• 00:04 Hi, I'm Ray Sheen, I'd like to talk about Float, Slack, Buffer.
• 00:09 Those terms are used interchangeably, but
• 00:11 they're an important concept in schedule planning.
• 00:14 To understand the concept of float, slack, or buffer,
• 00:18 let's work with the metaphor of a window.
• 00:21 Every task has a time window in which to accomplish the task.
• 00:25 The task cannot be started until the window opens and the task must be finished
• 00:30 before the window closes for the project to remain on schedule.
• 00:33 As long as the task is accomplished while the window is open,
• 00:36 the project will not be delayed.
• 00:38 Sometimes, the window is open much longer than is needed to complete the task.
• 00:43 You might have a six week window to do a five day task.
• 00:47 Sometimes, the window is open for
• 00:49 exactly the length of time that required to complete the task.
• 00:53 In that case, you may only have five days to do a five day task.
• 00:57 When that happens, you need to start the task the minute the window opens so
• 01:01 that you are done when the window closes.
• 01:03 Now I know, you're just thinking sometimes
• 01:06 the time window was smaller than the duration of the task, no it's not.
• 01:11 If you think that's the case during a project plan, you need to change the plan.
• 01:15 Don't start a project that you already know will fail.
• 01:19 If you don't have enough time, you should see the stakeholders to either
• 01:22 get the time boundary extended or the scope reduced or more resources added.
• 01:28 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'll be using float in the remainder of this lesson.
• 01:38 Be aware that the term buffer is use with a different definition
• 01:42 when you're doing the critical chain project scheduling.
• 01:46 So why do we care?
• 01:47 How do we use float to help us manage a project?
• 01:50 Float is created based upon the schedule plan that you've established for
• 01:53 the project.
• 01:55 If you have multiple paths of project activities,
• 01:57 you're almost guaranteed to have float.
• 02:00 The predecessor, successor relationships between the tasks and the estimates for
• 02:04 the task durations will determine how long each of those paths will take.
• 02:08 The longest path is the critical path and has no float.
• 02:12 All the other paths have float.
• 02:14 Changing the project schedule will change the float.
• 02:18 Either changing relationships or task estimates, or adding or deleting tasks.
• 02:22 If the duration of one of the paths changes, the float will change.
• 02:27 I use my float as a schedule reserve.
• 02:29 I want to know which paths to float and how much they have so
• 02:33 I know which paths have schedule reserve.
• 02:35 Paths with no float aren't by definition critical paths.
• 02:39 However, normally the majority of task are not on the critical path and
• 02:43 they have some float to work with.
• 02:45 So what do I do with that reserve?
• 02:48 I'll use it in one of three ways.
• 02:50 One approach is to position float immediately following a high risk task.
• 02:54 It maybe high risk because of estimate uncertainty or
• 02:57 maybe high risk because of technical difficulty.
• 03:00 It may even be high risk because of resource constraint.
• 03:02 Whatever the reason, the float is there to accommodate
• 03:05 any unplanned delay in that task without impacting any other task.
• 03:10 I'll also positioned some of the float right before a major milestone is due.
• 03:14 Often as you approach a milestone,
• 03:16 there will be little last minute problems at surface.
• 03:19 The float time is there to resolve those without impacting the milestone.
• 03:23 And finally, I use float to resolve resource over allocation problems.
• 03:27 If I have a resource over allocation at a particular time in the project schedule,
• 03:32 I try to use float to move one of the more active task at that time period
• 03:36 to a later time period when that task has time within its window.
• 03:41 There are two ways to calculate float, Total Float and Free Float.
• 03:45 Let's compare the two.
• 03:47 The Project Management Body of Knowledge, the PMBOK Guide defines Total Float as
• 03:51 the amount of time that a scheduled activity can be delayed or extended
• 03:55 from the earliest start date without delaying the project finish date or
• 03:59 violating a schedule constraint.
• 04:01 This is calculated for us with the critical path calculation and
• 04:04 its the latest time minus the earliest finish time for each task.
• 04:09 Of course, when those times are equal the answer is zero and
• 04:13 that task is then on the critical path, there is no float.
• 04:17 But as long as we have a positive number, there is Task Total Float.
• 04:21 The impact of exceeding the value of Task Total Float
• 04:24 is the cause of delay to the end date of the project.
• 04:27 Using up only a portion of the float will likely impact successor tasks,
• 04:32 although it will not impact the end of the project.
• 04:34 Now, we compare that Task Total Float with the other type of float, Task Free Float.
• 04:41 The Project Management Body of Knowledge, the PMBOK Guide, defines Free Float as
• 04:45 the amount of time that a schedule activity can be delayed without delaying
• 04:49 the early start date of any successor or violating a schedule constraint.
• 04:54 That means that this is the amount of float that a task can use
• 04:57 without impacting anything else from the project.
• 05:00 It's calculated by starting with the Task Total Float value and
• 05:03 subtracting the successor Task Total Float value.
• 05:07 If both task are on the same non-critical path with no other relationship
• 05:11 interventions, they will normally have the same Total Float value.
• 05:15 The Free Float value is then zero.
• 05:18 A delay of the first task impacts the start of the second.
• 05:21 The impact of exceeding the task Free Float is on the next task or
• 05:24 abandon the project.
• 05:26 Using up only a portion of task Free Float will not impact the schedule or
• 05:30 anything else in the project.
• 05:32 Now if you're planning on taking the PMP exam, be certain that
• 05:35 you understand the differences between these and how to calculate them.
• 05:41 The use of float is an excellent way to manage schedule risk in your project.
• 05:45 If your project has schedule constraints, it's imperative that you understand which
• 05:49 tasks and paths have float and which ones don't.
• 05:54 This is your schedule reserve.

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