Lead and lag time are common scheduling processes used when planning and executing a project. This post aims to clarify the distinctions between the two.
What is Lag Time?
Lag time refers to the amount of time that must pass after a predecessor activity has finished, before a successor activity can begin. Lag time can be thought of as a necessary delay before the start of the next activity. Lag time can occur in start-to-start, start-to-finish, finish-to-start, and finish-to-finish relationships.
Example of Lag Time
Assume we have a small project that is painting a room and hanging artwork on the walls. In this scenario, painting the room might take 1 day and will be one activity called Paint Walls. Hanging the artwork may take another day, and we can call that task Hang Artwork. However, we need to wait for the paint on the walls to dry before we can hang the artwork.
In this case, the Paint Walls task will have one day of lag built into the end to account for the time it takes for the paint to dry. The Hang Artwork task can begin on day 3, since the walls will be painted on day 1 and day 2 is the day accounted for the paint to dry.
What is Lead Time?
The time that a successor activity can be started earlier is referred to as lead time. That is to say the successor activity does not require the predecessor to be fully complete before the successor can be started. Lead time can only occur in finish-to-start relationships.
Example of Lead Time
Imagine you have a project planting trees. There is an activity to dig a hole for each tree to be planted which can be called Dig Holes, and will take 5 days. There is a second activity to plant a tree in each hole called Plant Trees which is also expected to take 5 days. The Dig Holes activity can start on day one, but the Plant Trees activity does not need to wait for all the holes to be dug before it can start.
Likewise, Plant Trees cannot start until at least some of the holes have been dug. So in this case, the Dig Holes activity can start on day 1, and the Plant Trees activity can start on day 2 giving 1 day of lead to its predecessor activity. This brings the total duration for the project to 6 days.
Key Differences Between Lead and Lag Time
Lag Time is the amount of time that must pass before a successor activity can start.
Lead Time is an acceleration of a successor activity.
Lead and Lag Together on the Same Project
Lets look at a quick example of a project that can have both lead and lag.
The project is to write a technical instruction manual. There are 3 activities associated with the project, they are Write Manual (5 days), Proof Read Manual (3 days), and Print Manuals.
The initial task Write Manual can begin on day 1. The proof reading process does not need to wait for the entire manual to be finished before it begins, so we can add a 2 day lead between the activities and start Proof Read Manual on day 3.
Finally, before the Print Manuals can be done, the customer must review and accept the manuals. This is a 5 day lag added to the end of the Proof Read Manuals activity. Once the customer has accepted the manuals, the Print Manuals task can begin.
I hope that helps illustrate the differences between lead and lag. Got something to add? Let me know in the comments below.