Introduction
This article discusses scheduling terminologies (Float, Leads, and Lags) which are commonly used for developing project schedule and followed by examples for fundamental understanding. This article is developed on the premise that the reader is having a basic understanding of project schedule network diagram, sequencing of activities, and logical relationships between the activities (e.g. Finish to Start, Start to Start, Finish to Finish, and Start to finish). This article is useful to PMP exam aspirants and to practitioners for developing robust project schedule and execute it accordingly.
Float, Leads, and Lags are very simple to understand and to apply in our project schedules. However, lack of appropriate knowledge can lead Project Manager (PM) to develop an inferior quality of project schedules and ultimately result in an ineffective execution of it.
Conceptual Understanding:
Precedence Diagramming Method
When it comes to the management of project schedules, the precedence diagram method helps PM to determine project activity flow. This flow of activity helps evaluate logical relationships or dependencies between the various project activities. That also includes evaluating relationships or dependencies between two dependent activities. The concept of Leads and Lags is critical in defining and evaluating these relationships. Leads and Lags worked as modifiers of these relationships or dependencies. Refer the below figure for Precedence Diagramming Method Relation Types.
Float
Among many project scheduling concepts, Float is one of the key concepts that represents the flexibility of the project schedule. Flexibility in terms of delaying or advancing the project activities. Float simply means freedom or breathing time available in the schedule for PM for executing the project activities. To understand this concept, refer the below-given example.
Activity A = 3 Days, Activity B = 2 Days, and Activity C= 3 Days. These activities are having Finish to Start Relationships. That means, when Activity A will finish, B will start and when B will finish, activity C will start. That means Activity C is dependent on the completion of Activity A and Activity B.
In the example given below, let's assume that Activity B is a complex activity and PM has kept 2 days of float to avoid failure of the overall project schedule. Here, this 2 days’ Float will work as an additional time or breathing time or freedom available to complete Activity B in case any undesired situation’s impacts on the execution of it. Please refer below-given illustrations for detailed understanding.
As shown in the illustration, let’s discuss the various scenarios of using this Float or additional time for executing Activity B and its impact on the successor Activity A and C.
Scenario 1: Start of Activity B immediately after finishing of Activity A
Let's assume that PM has decided to start the Activity B immediately after the completion of Activity A i.e. ES of activity B. If Activity B is completed in 2days as planned, then PM will be left with 2 days of freedom or breathing time.
In this case, PM can choose-
- To use 2 days of free time by advancing Activity C. i.e. ES of activity C and complete the project 2 days earlier (8 days) than planned (10 days) i.e. EF of Activity C or
- To do nothing and to wait for Activity C to start at its planned start time and to complete the project as planned (10 days).
Scenario 2: Delaying Activity B after Finishing Activity A.
After finishing Activity A, PM reviewed the work progress and found that the project activities are in control. PM chooses to delay the Activity B by 2 days after completion of Activity A. i.e. LS of Activity B. However, in this scenario, PM should have high confidence to complete the Activity B within its planned time of 2 days. Failure in finishing Activity B in 2 days will delay the Activity C and that ultimately may result in the delay of overall project schedule.
Scenario 3: Mixed Scenario
If the project activities are in control, PM can also choose to delay Activity B by 1 day i.e. LS of Activity B after the finishing of Activity A. If Activity B finishes in 2 days, then PM will be left with 1 day of float. PM can choose to advance the Activity C by 1 day i.e. ES of Activity C and finish the project 1 day earlier (9 days) or PM can choose to do nothing and wait for Activity C to be started at its planned start time.
Leads and Lags
Now, let’s understand this concept in detail. Leads and Lags are nothing but the type of floats (Freedom).
Lead
It is an activity relationship where successor activity is advanced in order to be conducted parallel to predecessor activity. That means the predecessor activity is still running and successor activity initiates. Let’s understand the below-given example.
Let us assume
- Activity A and Activity B has Finish to Start relationship.
- It takes 1 day to write 2 pages of report. Therefore, estimated time for finishing Activity A (Write a 12-page report) = 6 days.
- It takes 1 day to review 3 pages of report. Therefore, estimated time for Finishing Activity B (Review 12-page Report) = 4 days.
Therefore, for completing this project Activity A and B (Finish to Start relationship), PM will need 10 days. However, if PM chooses to advance the Activity B and start reviewing the completed pages of report from the 4th day of Activity A, then PM will be able to complete the project (Activity A & B) in a total of 7 days.
In a nutshell, when the predecessor activity is still running and successor activity starts, this is called Lead. The remaining time of Activity A when the Activity B starts is called Lead Time, or basically, the overlap time between Activity A and B is called Lead time. In the above example, Lead Time is 3 days.
This concept is applied as Fast Tracking technique for compressing the schedule.
Lag
It is an activity relationship where the successor activity is delayed to pass some time right after the completion of its predecessor activity. Lag describes delay or addition of time. Let’s understand the below-given example.
Let us assume
- Activity A and Activity B have Finish to Start relationship.
- It takes 1 day for plastering the wall. Therefore, estimated time for finishing Activity A = 1 day.
- It takes 1 day for painting the wall. Therefore, estimated time for finishing Activity B = 1 day.
Considering this scenario, PM will need 2 days for finishing this project Activity A and B. However, we cannot paint the wall until the cement plaster is set up and dried. Therefore, there will be a need of putting a mandatory delay after completing the Activity A to allow cement plaster to be set up.
Let us assume that PM chooses to put a lag of 1 day after finishing Activity A. In this case, PM will require 3 days to finish the project (Activity A and B).
In a nutshell, when predecessor activity finishes and if there is a need for delay or waiting period prior to starting the successor activity, then this is called Lag and the delay period is called Lag time. In the above example, Lag Time is 1 day.
Here it is quite worth to note that PM does not put lags in the project schedule without any proper justification as no one wants to delay the project. So, Lags should always be considered to satisfy some predefined requirement, condition, or strategic objective of project.
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