Activity Member: Lead and Lag in Schedule Development

PMBOK v8 Definition

A lead is the amount of time whereby a successor activity can be accelerated 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. In scheduling software, a lead is often represented as a negative value for lag. These concepts are applied within the Develop Schedule process, which belongs to the Schedule Management knowledge area of the Planning Process Group.

Why It Matters for the Exam

Lead and lag appear frequently in PMI exam questions testing your understanding of logical relationships and schedule compression techniques. Questions typically present a scenario where activities have dependencies with time offsets, and you must identify whether the situation describes a lead or a lag, or calculate the correct schedule adjustment.

Key Points to Remember (for the exam)

• Lead Definition: Time a successor activity can start before its predecessor finishes. Represented as a negative value for lag in scheduling software.

• Lag Definition: Time a successor activity is delayed after its predecessor finishes. Represented as a positive value.

• Finish-to-Start (FS) with Lead: The most common dependency type tested. Example: Landscaping starts 2 weeks before punch list completion = FS with 2-week lead.

• Finish-to-Start (FS) with Lag: Example: Technical writing team begins editing 15 days after writing begins = FS with 15-day lag.

• Common Confusion: Lead accelerates the schedule (negative lag), while lag delays the schedule (positive lag). Students often reverse the sign.

• Key Tool: Precedence Diagramming Method (PDM) is the tool used to model logical relationships including leads and lags.

• Main Output: The Project Schedule, which incorporates leads and lags in the network diagram.

Typical PMI Exam Example

You are scheduling a construction project. The "Pour Concrete" activity finishes on Day 10. The "Cure Concrete" activity requires 3 days after pouring before "Frame Walls" can begin. What type of relationship and time offset is this?

Answer: Finish-to-Start (FS) with a 3-day lag. The successor (Frame Walls) is delayed 3 days after the predecessor (Pour Concrete) finishes.

PMI Exam Traps

• Trap: Confusing lead with lag when the sign changes

  • Reality: Lead = negative lag (accelerates). Lag = positive lag (delays). A "2-week lead" means the successor starts 2 weeks earlier.

• Trap: Thinking lead applies only to Finish-to-Start relationships

  • Reality: Lead can apply to any logical relationship (FS, FF, SS, SF), though FS with lead is most common on the exam.

• Trap: Misreading the direction of the relationship

  • Reality: Always identify which activity is the predecessor and which is the successor. Lead/lag always modifies when the successor starts relative to the predecessor.

• Trap: Confusing lead/lag with schedule compression techniques (crashing, fast-tracking)

  • Reality: Lead is a modeling technique in the schedule. Fast-tracking involves adding leads to compress the schedule. They are related but not identical.

Important PMI Connections

Quick Review Questions

1. If Activity A has a Finish-to-Start relationship with Activity B and a 5-day lag, how many days after Activity A finishes does Activity B start?

2. In scheduling software, a lead of 3 days is entered as what value?

3. A project manager wants landscaping to start 2 weeks before the punch list is complete. What dependency and offset should be used?

4. What is the difference between a lead and a lag in terms of schedule acceleration or delay?

5. Which process produces the schedule that includes leads and lags as part of the network logic?

PMBOK v8 Reference

Section 5.2.3.2 - Develop Schedule: Tools and Techniques
(Logical relationships, leads, and lags are described in the Precedence Diagramming Method section)