Leads and Lags
Leads and lags are essential scheduling techniques used to adjust the timing relationships between tasks in a project schedule, allowing for more accurate modeling of real-world project conditions. Understanding and applying leads and lags enable project managers to fine-tune the schedule and optimize the sequence of activities. A "lead" is an acceleration of the successor activity, allowing it to start before its predecessor activity has fully completed. This is particularly useful when parts of the work can be overlapped without waiting for the entire predecessor task to finish. For example, in a construction project, landscaping work (successor) might begin before the building is fully completed (predecessor), provided that certain areas are accessible and safe to work on. Leads help in reducing the overall project duration by parallelizing activities where feasible. Conversely, a "lag" is a delay applied to the successor activity, enforcing a waiting period after the predecessor activity has completed before the successor can start. Lags are used to represent necessary delays due to various factors such as curing time for concrete, delivery lead times for materials, or other mandatory waiting periods. For instance, after painting a wall (predecessor), there might be a lag to allow the paint to dry before hanging decorations (successor). Applying leads and lags requires careful consideration to ensure that dependencies and constraints are accurately represented without introducing unrealistic overlaps or delays. Improper use can lead to scheduling inaccuracies and potential project risks. Therefore, project managers should document the reasoning behind leads and lags and regularly review them throughout the project lifecycle. By effectively managing leads and lags, project managers can achieve a more efficient schedule, identify opportunities to compress the project timeline, and better coordinate activities among different teams or contractors. It also aids in resource optimization by smoothing out peaks and valleys in resource usage. In summary, leads and lags are critical tools in schedule development that enhance the flexibility and accuracy of the project schedule. They allow for a realistic representation of task dependencies and timing, contributing to the successful delivery of the project within the desired timeframe.
Leads and Lags in Schedule Development
Why Leads and Lags are Important
Leads and lags are critical schedule development techniques in project management that help optimize project timelines and resource allocation. They directly impact a project's critical path, duration, and overall efficiency. Mastering leads and lags is essential for project managers to create realistic schedules, adapt to constraints, and deliver projects on time.
What are Leads and Lags?
Leads and lags are schedule compression techniques that modify the timing relationships between activities in a project network diagram:
Lead: A lead allows a successor activity to start before its predecessor activity finishes (an acceleration or overlap of activities). A lead is expressed as a negative number.
Lag: A lag requires a delay between a predecessor activity and a successor activity. A lag is expressed as a positive number.
How Leads and Lags Work
Leads and lags modify the four logical relationships (dependencies) between activities:
1. Finish-to-Start (FS): The most common relationship where a successor activity cannot start until a predecessor activity finishes.
- With a lead: FS-5 days means the successor can start 5 days before the predecessor finishes.
- With a lag: FS+5 days means the successor starts 5 days after the predecessor finishes.
2. Start-to-Start (SS): A successor activity cannot start until the predecessor activity starts.
- With a lead: SS-5 days means the successor can start 5 days before the predecessor starts (rarely used).
- With a lag: SS+5 days means the successor starts 5 days after the predecessor starts.
3. Finish-to-Finish (FF): A successor activity cannot finish until the predecessor activity finishes.
- With a lag: FF+5 days means the successor finishes 5 days after the predecessor finishes.
4. Start-to-Finish (SF): A successor activity cannot finish until the predecessor activity starts (rarely used).
Common Applications of Leads and Lags
- Construction projects: Using a lag for concrete curing time before beginning construction on top of a foundation
- Software development: Using a lead to allow testing to begin before coding is fully complete
- Manufacturing: Using a lag to account for paint drying time before assembly can continue
- Resource optimization: Using leads to maximize resource utilization by overlapping activities
Exam Tips: Answering Questions on Leads and Lags
1. Know the formulas: Memorize how leads (negative values) and lags (positive values) affect each dependency type.
2. Practice calculations: Be ready to calculate early start, early finish, late start, and late finish dates when leads and lags are applied.
3. Understand practical applications: Exams often present scenarios where you must determine when to apply leads or lags.
4. Watch for terminology confusion: Some questions may try to confuse leads (acceleration/overlap) with lags (delays).
5. Network diagram interpretation: Practice reading network diagrams with lead and lag notations.
6. Critical path impact: Understand how adding or removing leads and lags affects the critical path and float.
7. Remember the signs: Leads are represented as negative values (FS-10), while lags are positive values (FS+10).
8. Real-world constraints: When answering case-based questions, consider practical constraints like resource availability, technical requirements, and mandatory waiting periods.
9. Distinguish from other techniques: Be able to differentiate leads and lags from other schedule compression techniques like fast tracking and crashing.
10. Context matters: The same activity relationship might require a lead in one scenario and a lag in another based on project constraints.
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