Is the amount of time an activity can be delayed without taking away float from later activities?

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.

The critical path is the longest sequence of activities in a project plan which must be completed on time for the project to complete on due date. An activity on the critical path cannot be started until its predecessor activity is complete; if it is delayed for a day, the entire project will be delayed for a day unless the activity following the delayed activity is completed a day earlier.

The critical path is very useful in helping to manage any project. When the critical path has been identified, it can clearly be seen where effort cannot be compromised. If any of the activities on the critical path change, the end date of the project will be affected.

Critical Path Analysis

The work needed to complete the project needs to be broken down and all activities need to be defined. Once we know how long each activity will take, we can use this information to understand the duration of the project. A network schedule of activities needs to be completed. Each activity within the schedule is represented as follows:

Early Start – The earliest time that an activity can start according to the logical constraints.

Duration – The estimated time to undertake the activity.

Early Finish – The earliest time that an activity can finish according to logical constraints.

Late Start – The latest time that an activity can start according to logical constraints and without affecting the overall project duration.

Float – The time by which an activity may be delayed without affecting the overall project duration.

Late Finish – The latest time that an activity can finish according to logical constraints and without affecting the overall project duration.

In order to work out how long it will take to complete the sequenced work, we need to perform what is known as a forward pass. The early start of the first activity is zero and the early finish is calculated by adding the duration.

Early Start + Duration = Early Finish

This process is then carried through subsequent activities. Where an activity has two or more preceding activities it is the latest time which is transferred.

The example shown indicates that the quickest we can carry out the work we have identified is 28 days. However, what we don’t know is which of the activities are critical and if we have movement available (float) on any of the activities.

In order to establish the latest dates that an activity can commence without affecting the end date a back pass is performed. The early finish of the last activity in the network is transferred to the late finish. The duration is then subtracted from the late finish to obtain a late start. Where an activity has two or more succeeding activities, it is the earliest date that is transferred. This process is repeated throughout the network until all late start and finish dates have been identified.

Late Finish – Duration = Late Start

We can now work out what flexibility or float we have in the network. This is very important to the project manager as it will allow for decisions to be taken with the allocation of resources to maximise their utilisation.

There are two types of float:

Free Float – this is the amount of time a task can be delayed without affecting the succeeding task. This can be calculated by subtracting the Early Finish of an activity from the Early Start of its subsequent activity.

Total Float – this is the amount of time which an activity can be delayed without affecting the end date of the project.

Latest Finish – Earliest Finish = Float

Now that we have the early start and late start for each activity and have calculated the float available, we can work out the critical path through the network. The critical path is the series of activities within the network with zero total float. The critical path is shown in red in our example given.

There must be at least one critical path through any network. The path must be continuous but it may branch into a number of parallel paths.

It is important to remember that a critical path can change during a project, as actual durations and dates vary.

A helpful technique for controlling and managing the critical path is to invest some time in determining what is likely to go wrong in each of the main three project parameters, ie cost, time, quality. Although the critical path only reflects the time element, a compromise in either cost or quality can have a time impact.

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

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

Is the time an activity can be delayed without effecting the total float of preceeding and succeeding activity?

Is the amount of time a task can be delayed without it causing a delay to other tasks in the project?