9 Frequently Asked Questions on Lead and Lag

Table of Contents

Lead and Lag Demystified

There are many terms that perturb project management professionals. Lead and Lag are two such terms. I interact with many professionals on a regular basis. I often find that many of them are not fully aware of meaning of these terms. Even though they know the correct definitions of these terms, they use the terms incorrectly during conversations. I have also noticed that, these professionals do not have a clue about the application of Leads and Lags for the preparation project schedules.

I have already written an article to define Lead and Lag. In that article, I described the terms through explanatory diagrams and examples. I also explained how Leads and Lags can be used to modify basic Project Relationships.

This article extends my previous article. I have written this article to answer Frequently Asked Questions (FAQ) on Lead and Lag. While I have repeated the definitions of these terms in this article, it would be beneficial to read my previous article before reading this article. I think these two articles will overcome all the confusions that you might have regarding Lead and Lag.

Definitions

Lead

The amount of time whereby a successor activity can be advanced with respect to a predecessor activity.

PMBOK Guide

A Lead provides acceleration to the Successor Activity.

Lag

The amount of time whereby a successor activity is required to be delayed with respect to a predecessor activity.

PMBOK Guide

A Lag provides mandatory delay to the Successor Activity.

You can also look at Max Wideman’s Glossary for some other definitions of Lead & Lag.

Frequently Asked Questions

Is Lag same as Float?

– No. Lag is neither equivalent to Total Float nor to Free Float.
– Float depicts the flexibility in a Network Path. Float(s) come naturally after Project Dependencies are defined and Project Network Diagram is prepared. The Project Team can use Float(s) for optimizing the schedule.
– Lag is mandatory delay in the Project Schedule. The successor activity is required to wait for the duration of Lag.

Why should I put ‘mandatory delay’ in the Project Schedule? It would only extend the project duration.

The Lag or ‘mandatory delay’ is sometimes required in the project schedule. It is inherent to the Network Logic. It may be required due to physical constraints, stakeholders’ demands, or environmental reasons. Just look at a few examples of Lag to have a better understanding.

What will Project Team do during the Lag period? Will they sit idle?

No. Project Team can work on other project activities while they wait for the Lag period to complete.

Why should I use Lag? Why can’t I increase the duration of the Predecessor activity?

– Execution of Project Schedule Activities require Resources (Human Resources & other resources) and Finance. The Resource and Cost requirements are usually proportional to the duration of an activity.– Resource and Cost requirements for an activity would increase, if you increase the duration of an activity (Predecessor). The best practice is to schedule all activities as per the estimated duration.
– Lag is introduced only when a mandatory delay is required. Introduction of Lag does not impact the Resource or Cost requirements of an activity.

Is Lag same as Buffer?

No. Buffers are introduced in the Project Schedule to reduce Risk. They are also called time reserves. Buffers may or may not be used during the project execution. Moreover, actual duration of an activity may be more or less than the estimated duration. But a Lag will always be used as planned.

Is Lead same as Fast-Tracking?

No. Leads can be used to Fast-Track a project. But, fast-tracking can be done by changing the discretionary dependencies also.

Why is Lead required at all? One can remove the Dependency and start the successor activity early

No. Dependencies are defined because of the Network Logic. If there are any schedule changes in the predecessor activity(ies), they will be automatically affect the successor activity(ies). If one removes the dependencies, the schedule of successor activity will not automatically change due to changes in predecessor activity(ies). The Network Logic should be always maintained.

There is always some Float on Non-Critical Path. Are Leads and Lags required only on Critical Path?

No. Lead and Lag are modifiers to 4 types of logical relationships. They are as much part of Non-Critical Path and they are part of Critical Path. The network logical should always be maintained for all activities.

Why are four different relationships required when only one is enough?

– There 4 different Logical Relationships – Finish to Start, Start to Start, Finish to Finish and Start to Finish.
– Any Project Dependencies can be defined by using Finish to Start(FS) relationship with appropriate Lead or Lag.
– Logical Relationships are inherent to the Network Logic. The sequence of activities should be defined as it should be.
– Let us consider 2 activities – A and B. A has a duration of 3 days while B has a duration of 1 day. Mathematically, the logical relationships, B(S) = A(F) – 3d and B(S) = A(S) will produce the same result. But technically & physically the Logical relationship between A and B should be either FS or SS.
– Logical relationships are not mathematical in nature. They define the project schedule.

Over To You

Have you used leads and lags in your project? How have you been using them? Do you think you can make a schedule without using them?

Please leave a comment below.

0 & 1 Method Of Network Analysis

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3 Comments

DrPDG says:

July 6, 2014 at 8:38 AM

Hi again, Praveen,
While your answer is correct, there is an easier way to explain it….

You have two activities, A (predecessor) and B (successor)

You can then say that either the start or finish of A LEADS the start or finish of B by a predetermined amount of time

OR

You ca say that either the start or finish of B LAGS behind the start or finish of A by a predetermined amount of time.

Thus what determines whether that predetermined amount of time is a LEAD or LAG depends on whether the perspective is looking at it from the Predecessor or Successor point of view.

Hope this helps clarify in peoples minds what the differences are?

BR,
Dr. PDG, Jakarta, Indonesia
http://www.build-project-management-competency.com

1. Praveen Malik says:
  
  July 6, 2014 at 9:50 PM
  
  Thanks Dr. Paul. Your definitions are certainly simpler to understand.
  
Aisha Khalid says:

October 7, 2019 at 3:34 AM

Great explanation, uncomplicated and comprehensive. Thanks

9 Frequently Asked Questions on Lead and Lag