Last updated on July 17, 2016
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.
The amount of time whereby a successor activity can be advanced with respect to a predecessor activity.
PMBOK Guide 5th Edition
A Lead provides acceleration to the Successor Activity. You can also look at Max Wideman’s Glossary for some other definitions of Lead.
The amount of time whereby a successor activity is required to be delayed with respect to a predecessor activity.
PMBOK Guide 5th Edition
A Lag provides mandatory delay to the Successor Activity. You can also look at Max Wideman’s Glossary for some other definitions of Lag.
Frequently Asked Questions
– 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.
No. Project Team can work on other project activities while they wait for the Lag period to complete.
– 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.
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.
No. Leads can be used to Fast-Track a project. But, fast-tracking can be done by changing the discretionary dependencies.
No. Dependencies are defined because of the Network Logic. If there are any schedule changes in the predecessor activity(ies), they will be automatically reflected in the successor activity. 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 4 different Logical Relationships – Finish to Start, Start to Start, Finish to Finish and Start to Finish. 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.
– The Mathematics of relationships should not change the Network Logic.
I hope this post would have helped you in understanding the significance of Lead and Lag. Let me know if you have any other questions. You can post them as a comment and I will answer them.