It May Be Necessary to Use Lead or Lag Time Between Activities

PMBOK v8 Definition

Lead or lag time between activities may be necessary to support a realistic and achievable project schedule. Lead time is the amount of time a successor activity can be advanced with respect to a predecessor activity, while lag time is the amount of time a successor activity must be delayed with respect to a predecessor activity. The project management team determines the dependencies that may require a lead or a lag to accurately define the logical relationship, and the use of leads and lags should not replace schedule logic.

Why It Matters for the Exam

This concept appears frequently in PMI exam questions about the Schedule Management knowledge area, specifically in the Sequence Activities process. Questions typically test your ability to distinguish between lead and lag, identify when each is appropriate, and recognize that duration estimates do not include any leads or lags. You will encounter these concepts in both definition-based questions and scenario-based questions where you must apply lead/lag logic to network diagrams.

Key Points to Remember (for the exam)

Definition of Lead: The amount of time a successor activity can be advanced with respect to a predecessor activity. Lead is represented as negative time (e.g., FS-5 days means the successor starts 5 days before the predecessor finishes).
Definition of Lag: The amount of time a successor activity must be delayed with respect to a predecessor activity. Lag is represented as positive time (e.g., SS+15 days means the successor starts 15 days after the predecessor starts).
Critical Rule: Duration estimates do not include any leads or lags. Leads and lags are separate from activity duration and are applied to the logical relationship.
Do Not Replace Logic: The use of leads and lags should not replace schedule logic. They are refinements to logical relationships, not substitutes for proper sequencing.
Documentation Required: Activities and their related assumptions regarding leads and lags should be documented.
Common Example: A technical writing team may begin editing the draft of a large document 15 days after they begin writing it. This is shown as a start-to-start (SS) relationship with a 15-day lag.
Representation: Leads and lags can be represented in project schedule network diagrams.

Typical PMI Exam Example

A construction project requires that concrete must cure for 7 days before framing can begin on the same area. The relationship between "Pour Concrete" (predecessor) and "Begin Framing" (successor) is finish-to-start (FS) with a 7-day lag. If the project manager instead allows framing to begin 2 days before concrete pouring is complete, this would be a finish-to-start with a 2-day lead (FS-2).

PMI Exam Traps

Trap: Confusing lead with lag → Reality: Lead advances the successor (negative time), lag delays the successor (positive time). Remember: Lead = "leap forward," Lag = "lag behind."
Trap: Thinking duration estimates include leads or lags → Reality: Duration estimates do not include any leads or lags. They are separate parameters applied to logical relationships.
Trap: Using leads or lags to fix poor schedule logic → Reality: Leads and lags should not replace schedule logic. The logical relationship must be correctly defined first.
Trap: Confusing lag in scheduling with lagging indicators in monitoring → Reality: Lag in scheduling is a time delay between activities. Lagging indicators measure past performance. These are different concepts entirely.

Important PMI Connections

Related Concept	Relationship Type	Exam Attention Point
Sequence Activities Process	Process where leads/lags are applied	Part of Schedule Management knowledge area; defines the logical sequence of work
Logical Relationships (FS, SS, FF, SF)	Foundation for applying leads/lags	Leads and lags modify these relationships; know which relationship type is used with each lead/lag
Critical Path Method	Impacted by leads/lags	Leads and lags affect the forward and backward pass calculations and total float
Schedule Network Diagrams	Representation method	Leads and lags can be shown in network diagrams; understand how they appear visually

Quick Review Questions

A project has a finish-to-start relationship between Activity A and Activity B. If the project manager wants Activity B to start 3 days before Activity A finishes, what type of time adjustment is needed, and how is it represented?
True or False: When estimating the duration of an activity, you should include any lag time that will be applied to its predecessor relationship.
In a start-to-start relationship with a 10-day lag, if the predecessor starts on Day 1, when can the successor activity begin?
A project team uses lead time to accelerate the schedule. What is the primary risk of using lead time without proper schedule logic?
What must be documented when leads or lags are used in the project schedule?

PMBOK v8 Reference

Section 5 – Project Schedule Management, Sequence Activities Process (Planning Process Group, Schedule Management Knowledge Area)