Leads and Lags
A lead is the amount of time a successor activity can be advanced with respect to a predecessor activity. A lag is the amount of time a successor activity must be delayed with respect to a predecessor activity.
Explanation
Leads and lags are used to fine-tune the logical relationships between activities in a project schedule network diagram. They modify the standard dependency relationships (FS, SS, FF, SF) by either accelerating or delaying the successor activity relative to the predecessor.
A lead allows activities to overlap. For example, in a Finish-to-Start relationship with a 5-day lead, the successor activity can start 5 days before the predecessor finishes. This is sometimes called negative lag. Leads are often used to compress the schedule by allowing concurrent work where possible. However, leads introduce risk because the successor begins before the predecessor is fully complete.
A lag introduces a mandatory waiting period between activities. For example, after pouring concrete (predecessor), there might be a 3-day lag before the next construction phase (successor) can begin because the concrete needs time to cure. Lags represent real waiting time and cannot be shortened without changing the nature of the work. Both leads and lags are defined during the Sequence Activities process and are critical to accurate schedule modeling.
Key Points
- •Lead: accelerates (overlaps) the successor activity
- •Lag: delays the successor activity by a mandatory waiting period
- •Applied to any of the four dependency types (FS, SS, FF, SF)
- •Leads compress the schedule but add risk; lags represent real wait times
Exam Tip
A lead is sometimes called "negative lag." On the exam, if you see a negative value for lag, it means a lead. Leads compress the schedule; lags extend it.
Frequently Asked Questions
Related Topics
Sequence Activities
Sequence Activities is the process of identifying and documenting relationships among the project activities.
Precedence Diagramming Method (PDM)
The Precedence Diagramming Method (PDM) is a technique used to construct a schedule model in which activities are represented by nodes and are graphically linked by logical relationships to show the sequence of activities.
Finish-to-Start (FS) Dependency
A Finish-to-Start (FS) dependency is a logical relationship in which a successor activity cannot start until a predecessor activity has finished.
Schedule Compression
Schedule compression is a technique used to shorten or accelerate the project schedule duration without reducing the project scope.
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