Sometimes we need to either speed up the dependent activities or delay the dependent activities while keeping in the relationship as is. This is the purpose of Lead and Lag.
Lead:
Lead means ahead of time, go a little bit fast. In the case of FinishtoStart relationship, whatever time you want to lead, you need to reduce the start time of next activity by that time. For example, in the case of 2 days as lead time in FS relationship = ST (Start time)2 Days.

Lag: The lag time is added in the actual start date of the next activity. For example, in the case of building and painting, you say that first let me finish the product and then after a gap, I want to start painting. You need a gap of certain days before you start painting, so this is lagging time. A duration where no work is done. So leading refers to work done in parallel by 2 activities whereas lagging means giving a gap of some duration between 2 activities.

In Simple language Lead is something running ahead as speed and lag is an amount of gap introduced in between two activities.

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Materials in this video are based on the text, A Guide to the Project Management Body of Knowledge, (PMBOK® Guide) – Sixth Edition, Project Management Institute Inc., 2017.

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