Use of Leads and Lags
The use of leads and lags is a schedule compression technique that involves adjusting the timing relationships between tasks in a project schedule to optimize the overall duration. A lead is an acceleration of the successor activity, allowing it to start before its predecessor activity is fully complete. Conversely, a lag is a delay in the successor activity, meaning it starts after a certain period following the predecessor's completion. By strategically applying leads, project managers can overlap tasks that were previously sequential, thus shortening the project timeline without necessarily increasing resource allocation or costs. For example, in a software development project, testing (successor activity) can begin on completed modules before the entire coding phase (predecessor activity) is finished. By introducing a lead between coding and testing activities, the overall project duration is reduced. Careful adjustment of leads and lags requires thorough analysis to ensure that overlaps do not introduce undue risk or quality issues. It’s essential to maintain clear communication among team members to manage dependencies effectively. Misapplication of leads can lead to rework if the successor activity is started too early without adequate output from the predecessor. Using leads and lags as a compression technique is beneficial when there are constraints on adding resources or increasing costs. It relies on smarter scheduling rather than expanding the project budget. However, it should be applied judiciously to balance the benefits of time-saving with the potential risks to project quality and scope.
Use of Leads and Lags in Schedule Compression
What are Leads and Lags in Schedule Management?
Leads and lags are timing adjustments applied to schedule activities to optimize project timelines. A lead allows a successor activity to start before its predecessor finishes (overlap), while a lag requires a waiting period between the end of one activity and the start of its successor.
Why Leads and Lags are Important for Schedule Compression
As a schedule compression technique, properly applied leads and lags can significantly reduce project duration by:
• Creating parallel work paths where activities would normally be sequential
• Optimizing resource usage across the schedule
• Addressing practical constraints in activity relationships
• Providing flexibility in schedule management
How Leads and Lags Work in Practice
1. Implementing Leads:
• Example: A wall can be painted (successor) when 75% of drywall installation (predecessor) is complete, creating a 25% lead.
• Formula: Finish-to-Start with Lead = FS - X (where X is the lead duration)
2. Implementing Lags:
• Example: Concrete must cure for 3 days after pouring before construction can continue, creating a 3-day lag.
• Formula: Finish-to-Start with Lag = FS + X (where X is the lag duration)
3. Application in Network Diagrams:
• Leads are represented as negative values on dependency arrows
• Lags are represented as positive values on dependency arrows
Practical Application Steps:
1. Identify critical path activities that could benefit from overlapping
2. Analyze technical feasibility of implementing leads
3. Determine appropriate lag times based on technical requirements
4. Adjust the schedule network diagram to reflect modified relationships
5. Recalculate the critical path to verify compression benefits
Exam Tips: Answering Questions on Use of Leads and Lags
1. Recognize the terminology: Questions may refer to leads as "fast tracking at the activity level" or describe the concept rather than using the term explicitly.
2. Know the formulas:
• Lead: FS - X (negative value)
• Lag: FS + X (positive value)
3. Calculate schedule impacts: Practice determining how specific lead/lag values affect overall project duration.
4. Understand relationship types: Be comfortable with all dependency relationships (FS, SS, FF, SF) and how leads/lags apply to each.
5. Identify risks: Recognize that implementing leads increases technical risk and may require additional monitoring.
6. Differentiate from other techniques: Know how leads/lags differ from fast tracking (which applies to parallel paths) and crashing (which adds resources).
7. Remember practical limitations: Some activities simply cannot overlap beyond certain points for technical or logical reasons.
8. Context awareness: In exam scenarios, look for clues about which activities can safely overlap and which require mandatory waiting periods.
When facing questions about schedule compression, always consider leads and lags as your first option before moving to more resource-intensive options like crashing, especially when the question involves modest time reductions or mentions overlapping activities.
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