The amount of time an activity can be delayed without delaying the project finish date

Leads, lags and float are concepts used in schedule development process. The process of schedule development includes: identification of all activities, sequencing all activities based on dependency, estimating duration of each activity and finalizing the schedule. Leads, lags and float are used as part of activity sequencing process. All activities once sequenced will form a schedule network diagram. Let us first look at the definition of all these three attributes.

Float

Float (also known as slack) is the amount of time by which the start of an activity can be delayed without delaying the project completion time. Every task will have following set of start and finish time. 

• Earliest start time (ES) - The earliest time, an activity can start once the previous dependent activities are over.
• Earliest finish time (EF) - This would be ES + activity duration.
• Latest finish time (LF) - The latest time an activity can finish without delaying the project.
• Latest start time (LS) - This would be LF - activity duration.

Float time of an activity can be calculated by taking the difference between Late Start (LS) and Early Start (ES) OR between Late Finish (LF) and Early Finish (EF).

Float = LS-ES  OR 

=LF-EF

A positive float time indicates the flexibility we will have in delaying the specific activity without delaying the project completion time.

Typically, while doing scheduling, the critical path tasks will have zero float and the non-critical path tasks will have a positive float. That means non-critical path tasks can be delayed to certain extent without compromising on the project completion time. Float time information of tasks is very useful to the project team for taking scheduling decisions when there will be resource constraints.

Lag:

Lag is the amount of wait time between two tasks. Or in other words, lag is the amount of time by which a successor activity will be delayed. Lag can be used in all the four logical relationships in scheduling, such as Finish-to-start (FS), start-to-start (SS), finish-to-finish (FS) and start-to-finish (SF).

In below example, Task A and B have a Finish to Start (FS) relationship. Ideally both A and B should get finished on the 12th day. But when we insert a waiting of time of 2 days before B can start, then both A and B will get completed only on the 14th day.

Lead:

Lead is the amount of time a successor task can be accelerated. Lead can applied only on finish-to-start relationship between two activities. We can see the below example. In the below example, task B can start 2 days before the completion of task A. Hence the start of task B, which ideally would have been on 6th day, will not start on 4th day.

Conclusion:

Float, lead and lag are very important concepts and information for the scheduling team. A PMP training course ensures you get a hold of these concepts. These are used to optimally identify the dependencies and the associated constraints. Float information is useful in resource allocation when there are resource constraints. Lead is used for accelerating start of tasks (fast tracking) for reducing project timelines. Lag is used for ensuring that required idle or wait time after a task is appropriately provisioned.

Float is one of the very basic concepts essential for building network diagrams and in turn a project schedule. Further, there are different types of floats such as Free float, Total float, Project float, Interfering Float, Independent Float. As a Project Manager, you should have a clear understanding of these concepts in order to create efficient project schedules. So, lets see..

The term “Float” implies “Fluid”, which in turn implies “Flexibility“. In Project Scheduling, Float refers to the amount of scheduling flexibility. Float is also popularly called “Slack“.

The different types of float are

1. Total Float or Float
2. Free Float
3. Project Float
4. Interfering Float (INTF)
5. Independent Float (INDF)

Total Float 

Total float is the amount of time an activity can be delayed without delaying the project completion date.This is the type of Float that is commonly referred to as “Float”.

PMBOK defines Total Float as

“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.” 

So, Total Float is about flexibility at the project level. It is about the flexibility that an activity has in its execution without delaying the Project finish date.

Total Float Formula

Total float is calculated by subtracting the Early Start date of an activity from its Late Start date (Late Start date – Early Start date), or Early Finish date from its Late Finish date (Late Finish date – Early Finish date).

Total Float or Float = LS – ES or LF – EF

Free Float

PMBOK Defines Free Float as “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”

So, Free Float is about flexibility at the activity level. It is about the flexibility that an activity has in its execution without delaying its successor activity.

Free Float Formula

Free float is calculated by subtracting the Early Finish date of current activity from the Early Start date of its successor activity (ES of successor Activity – EF of current Activity).

Free Float = ES (of successor) – EF (of current) 

Total Float vs Free float

While Total Float is how much an activity can be delayed without affecting the project Finish date, Free Float is about how much an activity can be delayed without affecting its successor activity.

Project Float

This type of Float is not mentioned in the PMBOK, but is present in some reference books.

It is the amount of time a Project can be delayed without delaying the externally imposed project finish date by the customer, or the project finish date previously committed to by the Project Manager.

Free & Total Floats are about the time an activity can be delayed, while Project float is the amount of time a Project can be delayed.

Interfering Float (INTF)

Interfering Float is the amount of time a schedule activity can be delayed or extended from its early start date without delaying the project finish date. However, it should be understood that delaying an activity into interfering float will delay the start of one or more following non-critical activities. If an activity is delayed for the amount of the Free and Interfering Float, then its successor activities are critical.

Interfering Float Formula

Interfering Float = Total Float – Free Float

Independent Float (INDF)

Interfering Float is the maximum amount of time an activity can be delayed without delaying the early start of the succeeding activities and without being affected by the allowable delay of any predecessor activity.

Independent Float Formula

Independent Float (INDF) = Earliest Successors’ Early Start – Earliest Predecessors’ Late Finish – Activity’s duration

when the INDF is a negative value, we set the value to zero.

Is the amount of time that an activity can be delayed without delaying the project?

What is the amount of time that a schedule activity can be delayed without delaying the early start of any successor activity or violating a schedule constraint?

Is the amount of time an activity can be delayed without impacting the early start date of its successor?

Which is the amount of time an activity can be delayed?