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Project schedules are often modified to implement risk response plans.  Leads, lags, float and levelling are used to understand and manage elements of project risk.

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Quick reference

Leads, Lags, Float, and Leveling

Project schedules are often modified to implement risk response plans.  Leads, lags, float and levelling are used to understand and manage elements of project risk.

When to use

Resource over-allocation and task float are the result of project planning.  When the initial plan is created, leads, lags, and float can be used to resolve over-allocation problems through levelling.  Whenever a project is replanned, float and resource allocation issues are again examined and leads and lags are again used to modify the schedule.  When a project encounters a major problem, leads and lags are often used to change the schedule to accommodate the problem.  Although, float and levelling issues must be monitored to ensure they do not create additional problems.

Instructions

These are factors to be considered when planning a project.

Leads and lags

Lead: “The amount of time whereby a successor activity can be advanced with respect to a predecessor activity.” PMBOK® Guide

Lag: “The amount of time whereby a successor activity will be delayed with respect to a predecessor activity.” PMBOK® Guide

Leads and lags are modifications to the start date of project tasks or activities from the normal default of “early start.”  When project schedules are initially created, they normally use the “early start” approach which means that a task is scheduled to start as soon as all predecessor tasks have completed.  Inserting a pause between the completion of the predecessor tasks and the start of the successor task is referred to as a lag.   This is often done to align task schedules with resource availability or to implement a risk mitigation action.

A less common approach is to start a task prior to the completion of the predecessor tasks.  Normally this is used to allow some task work to be initiated so as to accelerate the task completion.  This can be a high risk approach if the final result of the predecessor task forces a change to work that has already been completed on the successor task. 

When using a lead or lag, the amount of lead and lag is specified and the delay or overlap is reflected in the schedule.  Examples are shown below.

 

Definitions are 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, Page 709.

Float

Total Float: “The amount of time that a schedule 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

Float is an artifact of the project schedule.  Float can be determined once the critical path is calculated.  Therefore, to determine float you must have a network diagram with all tasks and a schedule estimate for all tasks.  Float represents the unassigned time on the non-critical paths in the project.  Float is a project manager’s best friend when problems arise.  Knowing which tasks have float and how much float allows the project manager to prioritize work without impacting the project end date.

There are two ways to describe float, Total Float and Free Float.  Total Float is the value that is determined when you do the float calculation in a critical path analysis.  It represents the amount of unassigned time on that path.  Although calculated for a task, it is a path-level measurement.   That time can be positioned anywhere within that path, based upon the project risk management decisions.  Once Total Float has been used with one task on a path, that float is no longer available for other tasks on that path.  You can position float in the middle of a path by inserting a lag between tasks.

Free float represents the flexibility of an individual task.  It is the float available to a task before it impacts any other task, including those on its path.  Unless a task is the final task on a non-critical path, or a lag has been added after its completion, tasks will not have Free Float, even though they may have a high value for Total Float.  Examples are shown below.

Definitions are 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, Pages 707 and 725.

Resource leveling

Resource Leveling: “A resource optimization technique in which adjustments are made to the project schedule to optimize the allocation of resources and which may affect critical path.” PMBOK® Guide

This technique adjusts task start and finish dates so as to reduce the amount of resources needed during a time of over-allocation.  The start of the task can be delayed.  The start can occur as planned but the end date may be delayed allowing a reduction in the daily level of resources needed to do the task.  At times the task is broken into multiple sub-tasks that are scheduled to start and stop based upon resource availability. 

This technique is often attempted with software.  However, many software programs do not optimize for early schedule completion when resource leveling.  To do this, critical path tasks must be fully resourced.  Then the start and finish dates for the remaining non-critical path tasks are modified to position the work when resources are available.  If there are multiple over-allocated resources, this becomes very difficult to manage.

In the example shown below, resources are over-allocated during weeks 5, 6, and 7.  The project requires 6 people and only 5 are available.  The leveling was accomplished by changing the end date of Task 3 which delayed the end of Task 3 from week 7 to week 9, which reduced the number of people to fit within the available resources.

  

Definitions are 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, Page 719. PMBOK is a registered mark of the Project Management Institute, Inc.

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  • 00:04 Hi, I'm Ray Sheen, I'd like to talk about leads, lags, leveling and float.
  • 00:10 It's likely that you will have some PMP questions
  • 00:13 that will involve these calculations.
  • 00:17 Let's start with leads and lags.
  • 00:20 These are schedule modifications that are used to adjust relationships between two
  • 00:24 tasks or activities.
  • 00:25 This is usually done to lower risk or
  • 00:27 resolve some type of conflict, such as a resource conflict, or
  • 00:31 to align activities with an intermediate or external milestone.
  • 00:34 A typical relationship between tasks, the predecessor to successor relationship,
  • 00:39 is modified to create delays or overlaps.
  • 00:43 The Project Management Body of Knowledge,
  • 00:45 the PMBOK Guide, defines a lead as the amount of time whereby a successor
  • 00:50 activity can be advanced with respect to a predecessor activity.
  • 00:54 As you can see in the diagram, there's an overlap between the two tasks.
  • 00:58 This will accelerate the overall project.
  • 01:01 The PMBOK Guide defines a lag as the amount of time whereby a successor task
  • 01:05 will be delayed with respect to a predecessor activity.
  • 01:09 In the diagram, you can see that there is a gap between
  • 01:12 when the predecessor task ends and the successor task starts.
  • 01:16 This will extend the total duration time for these tasks.
  • 01:20 Let's talk about resource leveling.
  • 01:22 This is often done by applying lags to tasks
  • 01:25 in order to resolve a resource conflict.
  • 01:28 The Project Management Body of Knowledge,
  • 01:29 the PMBOK Guide, defines this as a resource optimization technique
  • 01:35 in which adjustments are made to the project schedule.
  • 01:37 To optimize the allocation of resources and which may affect critical path.
  • 01:43 In this case, we're resolving a resource problem by changing the schedule.
  • 01:47 There are other ways to resolve resource issues, but
  • 01:50 this method relies on schedule changes.
  • 01:53 Resource leveling is needed when we have a resource that is over-allocated.
  • 01:57 We change the schedule so
  • 01:59 that the tasks fit within the available resource constraint.
  • 02:03 Let's take a look at this example.
  • 02:04 There are three tasks that all require the same resource.
  • 02:08 Task 1 starts on weak one and requires two people for five weeks.
  • 02:12 Task 2 starts on week three and requires three people for eight weeks.
  • 02:17 And Task 3 starts on week five and requires three people for three weeks.
  • 02:21 The problem is that there are only five people available.
  • 02:24 In week five, we need eight people and in weeks six and seven,
  • 02:27 we need six people but there are only five.
  • 02:30 We need to level the resources in order to resolve this problem of over-allocation.
  • 02:36 We'll do this by changing start dates and end dates and durations.
  • 02:40 We add a lag of one week to the start of week three.
  • 02:43 This resolves the problem in week five,
  • 02:45 because now task three won't start until week six.
  • 02:49 However, we still have a problem in week six and seven, and
  • 02:52 now we have an added problem in week eight.
  • 02:55 So, we then extend the duration of Task 3 from three weeks to five weeks.
  • 03:00 Which reduces the number of people needed each week to just two people in the first
  • 03:04 four weeks of the task and only one person in the final week.
  • 03:08 This has now resolved the resource over-allocation problem.
  • 03:11 We leveled the resources by moving resource requirements
  • 03:15 into the available time that we had in weeks eight, nine and ten.
  • 03:19 Let's now talk about float.
  • 03:21 By adding lags to task relationships,
  • 03:23 we are often repositioning the total task float along a path.
  • 03:27 Most project management software will default to starting every task in a path
  • 03:31 as soon as possible.
  • 03:33 However, you may want to delay a task to reduce risk in some manner.
  • 03:38 If there is float along a path, you can use lag to position that
  • 03:41 float following whatever task you choose along that path.
  • 03:45 I often use this technique to position float right after a high risk task,
  • 03:50 so that I already have time available to resolve any problems
  • 03:53 that occurred in that task.
  • 03:55 Let's take a look at this.
  • 03:57 There are two categories of float.
  • 03:59 According to the PMBOK Guide, task total float is
  • 04:02 the amount of time that a scheduled activity can be delayed or
  • 04:05 extended from its early start date without delaying the project finish date.
  • 04:10 Or violating a schedule constraint.
  • 04:12 Looking at our picture, there is no float for the red tasks.
  • 04:16 However, the blue tasks both will indicate a total float of one week.
  • 04:21 Either of those tasks could be delayed by one week
  • 04:24 without impacting the project end date.
  • 04:26 However, both tasks cannot be delayed by a week,
  • 04:28 that would add up to two weeks of delay.
  • 04:30 Only one of them can absorb the delay.
  • 04:33 Task total float is related to the completion date of a project.
  • 04:37 It's the big picture look at float.
  • 04:39 The other type of float is task free float.
  • 04:42 The PMBOK Guide defines this as the amount of time a schedule activity can be delayed
  • 04:47 without delaying the early start date of any successor or
  • 04:50 violating a schedule constraint.
  • 04:52 Free float doesn't look at the end of the project,
  • 04:55 it only looks as far as the next task.
  • 04:58 So in our example, the first blue task has no free float.
  • 05:02 Because if it is delayed even one day it delays the second blue task.
  • 05:06 However, the second blue task has one week of free float.
  • 05:09 It can be delayed by one week before it impacts a red task.
  • 05:13 Free float then represents the flexibility or
  • 05:15 buffer a task has before it impacts anything else in the project.
  • 05:20 It should be obvious that free float can never be greater than total float, and
  • 05:24 when a task is on the critical path, both free float and total float are equal to 0.
  • 05:30 Critical path calculations will give you the total float amount for each task.
  • 05:34 Use leads and lags to modify a task's start and end date.
  • 05:38 This will position increments of that total float along a path and
  • 05:42 give tasks on the path free float.
  • 05:44 The sum of all the free float on a path is equal to the total float for that path.
  • 05:51 Leads, lags, leveling and float are all concepts used
  • 05:55 to manage a project's schedule so as to lower overall risk.
  • 06:00 You'll have PMP exam questions that relate to some or all of these topics.

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