Leads and Lags in Schedule Logical Relationships

PMBOK v8 Definition

A lead is the amount of time whereby a successor activity can be advanced with respect to a predecessor activity. A lag is the amount of time whereby a successor activity will be delayed with respect to a predecessor activity. Leads and lags are used to accurately define the logical relationship between activities in the project schedule, and they should not replace schedule logic. Duration estimates do not include any leads or lags.

Why It Matters for the Exam

Leads and lags appear frequently in PMI exam questions testing your understanding of schedule development and sequencing logic. You will encounter them in situational questions where a project manager must determine whether to accelerate or delay a successor activity, and in definition questions that ask you to distinguish between lead and lag. The exam also tests your ability to identify the correct dependency type (FS, SS, FF, SF) combined with lead or lag values.

Key Points to Remember (for the exam)

• Lead Definition: Time a successor activity can be advanced with respect to its predecessor (often represented as a negative value in scheduling software)
• Lag Definition: Time a successor activity will be delayed with respect to its predecessor (always a positive value)
• Lead Example: Landscaping can start 2 weeks before punch list completion → Finish-to-Start (FS) with 2-week lead
• Lag Example: Editing begins 15 days after writing starts → Start-to-Start (SS) with 15-day lag
• Critical Rule: Leads and lags should not replace schedule logic
• Duration Rule: Activity duration estimates do not include any leads or lags
• Documentation: Activities and their related assumptions should be documented

Typical PMI Exam Example

A technical writing team begins writing a large document. The editing team can begin editing 15 days after writing starts. The project manager creates a Start-to-Start (SS) relationship with a 15-day lag. Later, the project manager realizes the editing team could start 5 days earlier if resources are available. This requires reducing the lag from 15 days to 10 days, not changing the logical relationship.

PMI Exam Traps

• Trap: Confusing lead with lag → Reality: Lead advances a successor (negative value); lag delays a successor (positive value)
• Trap: Thinking leads and lags are part of activity duration estimates → Reality: Duration estimates exclude leads and lags; they are separate schedule attributes
• Trap: Using leads or lags to fix poor schedule logic → Reality: Leads and lags should never replace proper schedule logic; they only refine logical relationships
• Trap: Confusing lead with fast-tracking → Reality: Lead is a planned time advancement in the schedule; fast-tracking is a compression technique that changes the relationship

Important PMI Connections

Quick Review Questions

1. A construction project has landscaping scheduled to start 2 weeks before the punch list completion. What dependency type and lead/lag value should be used?

2. A technical writing team begins editing 15 days after writing starts. Is this a lead or a lag? What dependency type applies?

3. If a successor activity has a finish-to-finish (FF) relationship with a 5-day lag, what does this mean for the schedule?

4. Should activity duration estimates include any leads or lags? Why or why not?

5. A project manager wants to advance a successor activity by 3 days. Should they use a lead or a lag? What value would appear in scheduling software?

PMBOK v8 Reference

Section 5 – Tools and Techniques (Leads and Lags) Section 2 – Project Management Performance Domains (Step 3: Estimate effort and duration)