Leads and Lags in Project Scheduling: PMP Exam Concepts Clarified

Leads and Lags: Small Adjustments, Big Exam Impact

Leads and lags are modifiers applied to the logical relationships between project activities. They seem simple — a lead accelerates the start of a successor, a lag delays it — but PMP exam questions on this topic require precise understanding of how these modifiers interact with different dependency types and affect the critical path.

What Is a Lead?

A lead is the amount of time a successor activity can be advanced with respect to a predecessor activity. In practical terms, it allows the successor to start before the predecessor finishes (in a finish-to-start relationship).

Lead Example

Activity A (pour foundation) has a finish-to-start relationship with Activity B (begin framing). Normally, B waits until A finishes. But if the last section of foundation can cure while framing begins on the first section, a 2-day lead on the FS relationship lets B start 2 days before A finishes.

Leads are expressed as negative values in scheduling software: FS -2 days means a 2-day lead.

What Is a Lag?

A lag is the amount of time a successor activity must be delayed with respect to a predecessor activity. It represents mandatory waiting time built into the schedule.

Lag Example

Activity C (pour concrete) has a finish-to-start relationship with Activity D (apply sealant). The concrete needs 5 days to cure before sealant can be applied. This 5-day lag is added to the FS relationship: FS +5 days.

Unlike leads, lags add time to the schedule and can extend the critical path.

How Leads and Lags Affect the Network Diagram

When calculating the critical path, leads reduce the effective duration between activities, and lags increase it. The forward and backward pass calculations must account for these modifiers:

• With a lead: ES of successor = EF of predecessor - lead value.
• With a lag: ES of successor = EF of predecessor + lag value.

This means leads can shorten the critical path (or at least the path through those activities), while lags can lengthen it. A lag on a critical path activity directly increases the project duration.

Leads and Lags with Non-FS Relationships

Although most examples use finish-to-start, leads and lags apply to all four dependency types:

• SS with lead: Successor can start before the predecessor starts by the lead amount.
• SS with lag: Successor must wait a specified time after the predecessor starts.
• FF with lead: Successor can finish before the predecessor finishes by the lead amount.
• FF with lag: Successor must wait after the predecessor finishes before it can also finish.

PMP Exam Pitfalls

Common traps in PMP questions about leads and lags:

1. Confusing leads with fast tracking: Fast tracking changes the relationship type or removes a dependency. A lead modifies an existing relationship without changing its type. Both can allow parallel work, but they are different concepts.
2. Using leads to mask poor planning: PMI generally discourages excessive use of leads because they increase risk. If the predecessor's output changes, the successor may need rework.
3. Forgetting that lags consume time: A common error is treating lag as free time. It is scheduled time that extends the project duration if it falls on the critical path.
4. Negative lag vs. lead: Some questions use "negative lag" as a synonym for "lead." They mean the same thing.

Practical Application

In practice, leads and lags are used to model real-world constraints that pure dependency logic cannot capture. Curing time, shipping delays, approval waiting periods, and regulatory review periods are all modeled as lags. Construction overlaps, phased handoffs, and parallel testing are modeled as leads.

Understanding these modifiers is essential for accurate schedule development and for answering PMP questions about the Sequence Activities process. Explore our scheduling topic guides for more practice.