Leads and Lags
Leads and lags are scheduling techniques used to adjust the timing relationships between activities in a project schedule. They modify the logical relationships defined during activity sequencing to reflect real-world constraints and optimize the schedule. - **Lead:** A lead is an acceleration of the successor activity. It allows the successor activity to start before the predecessor activity finishes. For example, in software development, testing (successor) can begin two weeks before coding (predecessor) is complete. This is represented as a negative value in scheduling software. - **Lag:** A lag is a delay in the successor activity. It causes a delay after the predecessor activity has finished before the successor can start. For example, after pouring concrete (predecessor), there may be a mandatory cure time of three days (lag) before construction can continue (successor). Incorporating leads and lags helps in creating a more accurate and flexible project schedule. They account for overlaps and delays that occur due to practical constraints, resource availability, or strategic decisions. Proper use of leads and lags can shorten the project duration without compromising the logical sequence of activities. Project managers must document leads and lags thoroughly to maintain clarity in the schedule and ensure that all stakeholders understand the timing adjustments. Misuse or overuse can lead to confusion and scheduling errors. Therefore, leads and lags should be applied judiciously and reviewed regularly as the project progresses to ensure they remain valid and beneficial to the project timeline.
Leads and Lags: Definition, Importance, and Exam Preparation Guide
What are Leads and Lags?
Leads and lags are schedule compression techniques used in project scheduling to adjust the relationship between activities. Lead is the amount of time whereby a successor activity can be advanced with respect to a predecessor activity. Lag is the amount of time whereby a successor activity is delayed with respect to a predecessor activity.
Why are Leads and Lags Important?
Leads and lags are critical components of project scheduling because they:
- Allow for realistic representation of dependencies between activities
- Help optimize project schedules by identifying opportunities to overlap or delay activities
- Enable more accurate modeling of real-world constraints
- Support schedule compression when needed
- Provide flexibility in managing resource allocation
How Leads and Lags Work
Leads (Negative Lag):
A lead allows the acceleration of the successor activity. For example, a 5-day lead on a Finish-to-Start relationship means the successor can start 5 days before the predecessor finishes.
Example: In construction, foundation drying (predecessor) and wall framing (successor) have a FS-5 relationship, allowing framing to begin 5 days before the foundation fully cures.
Lags:
A lag creates a delay between activities. For example, a 3-day lag on a Finish-to-Start relationship means the successor starts 3 days after the predecessor finishes.
Example: In software development, testing (successor) may begin 2 days after coding (predecessor) is complete, represented as FS+2.
Types of Relationships with Leads and Lags:
1. Finish-to-Start (FS): Most common, where successor starts after predecessor finishes
2. Start-to-Start (SS): Successor starts after predecessor starts
3. Finish-to-Finish (FF): Successor finishes after predecessor finishes
4. Start-to-Finish (SF): Successor finishes after predecessor starts
Exam Tips: Answering Questions on Leads and Lags
1. Understand the notation: Recognize that leads are shown as negative values (FS-5) and lags as positive values (FS+3).
2. Calculate schedule impacts: Be ready to determine how a lead or lag affects start and finish dates of activities.
3. Dependency types: Know how leads/lags function differently with each dependency type (FS, SS, FF, SF).
4. Critical path analysis: Understand how leads and lags can affect the critical path and overall project duration.
5. Schedule compression: Identify when leads can be used as a fast-tracking technique to reduce project duration.
6. Common scenarios: Familiarize yourself with typical scenarios where leads and lags are applied:
- Curing/drying time (lag)
- Overlapping design and implementation (lead)
- Administrative delays (lag)
- Partial dependencies (lead)
7. Network diagrams: Practice interpreting and drawing network diagrams that include leads and lags.
8. Terminology clarity: Some exam questions may try to confuse leads and lags - remember leads accelerate and lags delay.
9. Mathematical calculations: Practice schedule calculations with networks containing various leads and lags.
10. PDM focus: Recognize that leads and lags are primarily used in Precedence Diagramming Method (PDM).
When studying for the exam, create practice scenarios with different combinations of dependencies and leads/lags to ensure you fully grasp how they affect project schedules.
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