Lead & Lag

Lead and lag are the terms used to define a relationship between the tasks on a project. Both are scheduling terms used with relationships like Finish to Start, Start to Start. We will see below with simple examples.

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Lead

A Lead is the amount of time that successor activity is ahead of from its predecessor activity. For example in a construction project, it is may be desired to start working on landscaping 4 weeks before the furniture purchase. This will be shown on scheduling diagram like below

Lead between the tasks

This type of interlinked activity sequence is known as finish to start and is most often used type of sequence in project scheduling. In simple words we can say that lead time is 4 weeks.

This type of interlinked activity sequence is known as finish to start and is most often used type of sequence in project scheduling. In simple words we can say that lead time is 4 weeks.

In this scenario, these 5 days’ time that will be before the design finishes when the development starts is called as lead time.

Example for Lead

Let’s take an example of a project involving software development. We might want to start with programming of hardware screen where software will run as the programming itself is dependent on technical design including dimensions of screen. So both activities can definitely not start simultaneously. When we go deep into activity plan we know that technical design will take 20 days but after 15 days most of the activities are in final stages. Most importantly the information required for programming is concluded so we can start programming from there on. Hence overall duration of a project will be reduced.

Lag

Completely opposite to Lead, Lag is the amount of time successor activity is delayed from its predecessor activity.

For example time duration for first activity is 2 days and then there is a gap of 3 days before you start next activity so you can say that there is lag of 3 days. It will be shown on scheduling diagram as Finish to Start.

Let us look at below snip, here we have to wait to start Paint work after Plastering is finished. This wait is mandatory as Platser will take time to set or cure.

Lag between the tasks

Similarly we can also have a case like you may start editing a book after 10 days you begin writing it. It will be shown as start to start sequence as below

Similarly we can also have a case like you may start editing a book after 10 days you begin writing it. It will be shown as start to start sequence as below

Example for Lag

Let’s consider another example from a construction project. It is only logical to first prepare concrete and get the foundation work completed only then we can start working on rest of construction activities. But there is one problem, as foundation requires curing of concrete for at least 3 days, remaining work can’t be initiated. These 3 days’ time can be termed as lag which can’t be avoided. So it is important to understand the logical dependencies of activities for precise project planning.

Wrap Up

Lead and lag both are important concept and must be considered in project schedule development. Lead is the amount of time that successor activity is ahead of from its predecessor activity. Whereas Lag is the amount of time successor activity is delayed from its predecessor activity.


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