Leads and Lags
Leads and lags are scheduling techniques used in project management to adjust the timing between dependent activities, allowing for more precise control over the schedule. These concepts are integral to schedule network analysis as they help model real-world scenarios where activities may not follow each other in a strict, sequential manner. - **Lead:** A lead is an acceleration of the successor activity. It allows the successor activity to start before the predecessor activity has fully completed. For example, in a software development project, testing (successor) might begin two days before coding (predecessor) is complete, representing a lead. Leads are often used to compress the schedule or to reflect overlaps between activities that can occur simultaneously to some extent. - **Lag:** A lag is a delay in the start of the successor activity after the predecessor activity has finished. It introduces a waiting period between activities. For instance, after painting a wall (predecessor), there may be a required drying time of 24 hours before installing fixtures (successor), representing a lag. Lags are used to model delays that are out of the project team's control or to incorporate necessary waiting times into the schedule. Incorporating leads and lags into the project schedule allows for greater flexibility and realism. They enable project managers to create more accurate and efficient schedules by accounting for overlaps and delays inherent in project activities. Proper application of leads and lags can improve project timelines without compromising the quality of work or overloading resources. However, they must be used judiciously, as inappropriate use can introduce risks such as resource conflicts or unrealistic timelines. Understanding and managing leads and lags are essential skills for effective schedule network analysis and successful project delivery.
Leads and Lags in Schedule Network Analysis
Understanding Leads and Lags in Project Scheduling
Leads and lags are critical time adjustment techniques in schedule network analysis that enable project managers to accurately model the relationships between activities. These concepts are essential components of the Project Management Professional (PMP) and PMI Scheduling Professional (PMI-SP) certifications.
What Are Leads and Lags?
Lag is a delay in the successor activity. For example, if you need to wait 3 days after pouring concrete before you can begin construction, those 3 days represent a lag.
Lead is an acceleration of the successor activity, allowing it to start before its predecessor finishes. For example, if a successor activity can start when its predecessor is 75% complete (rather than 100%), this represents a lead.
Why Are Leads and Lags Important?
1. Realistic Scheduling: They help create more realistic project schedules by accounting for real-world timing constraints.
2. Resource Optimization: Proper use of leads can compress schedules and improve resource utilization.
3. Dependency Management: They provide nuanced control over the four types of dependencies (Finish-to-Start, Start-to-Start, Finish-to-Finish, Start-to-Finish).
4. Schedule Flexibility: They introduce options for adapting to changing project circumstances.
How Leads and Lags Work in Practice
Examples of Lags:
• Concrete curing (must wait 3 days after pouring before building on it)
• Document approval cycles (must wait for review before proceeding)
• System testing (must wait for testing completion before deployment)
Examples of Leads:
• Starting to order materials when design is 80% complete
• Beginning training while system development is still underway
• Starting documentation before a feature is fully implemented
Mathematical Representation
In network diagrams and scheduling tools:
• Lag is represented as a positive number: FS+3 means a Finish-to-Start relationship with a 3-day lag
• Lead is represented as a negative number: FS-2 means a Finish-to-Start relationship with a 2-day lead
Application in Different Dependency Types
1. Finish-to-Start (FS): Most common; successor starts after predecessor finishes (with adjustment for lead/lag)
2. Start-to-Start (SS): Successor starts after predecessor starts (with adjustment for lead/lag)
3. Finish-to-Finish (FF): Successor finishes after predecessor finishes (with adjustment for lead/lag)
4. Start-to-Finish (SF): Successor finishes after predecessor starts (with adjustment for lead/lag)
Exam Tips: Answering Questions on Leads and Lags
1. Know the Definitions: Understand that lead is an acceleration (negative time value) and lag is a delay (positive time value).
2. Watch for Context: Pay attention to whether the question is asking about compressing or extending a schedule.
3. Calculate Carefully: When working with network diagrams, add lags and subtract leads when calculating timing.
4. Consider Dependency Types: Remember that leads and lags can apply to all four dependency types, not just Finish-to-Start.
5. Look for Hidden Clues: Questions may describe a lead or lag situation but use different terminology.
6. Understand the 'Why': Be able to identify legitimate reasons for applying leads and lags (technical requirements, resource constraints, best practices).
7. Practice Notation: Become comfortable with notation like FS+3d (3-day lag) or SS-2d (2-day lead).
8. Remember Practical Applications: Connect concepts to real-world scenarios to better remember them during exams.
Common Exam Scenarios
1. Critical Path Calculation: Questions may ask how a specific lead or lag affects the critical path.
2. Schedule Compression: You might need to identify where leads could safely be applied to compress a schedule.
3. Dependency Analysis: Questions may ask which type of relationship with what lead/lag would be most appropriate in a given scenario.
4. Risk Impact: How changes to leads and lags might impact project risk exposure.
By mastering leads and lags, you gain powerful tools for creating realistic, optimized project schedules. These concepts are heavily tested on PMI exams because they represent fundamental skills for effective schedule management.
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