In Primavera P6, the relationship of an activity refers to the logical dependency or sequence that one activity has with another. These relationships define how activities are connected in a project’s schedule and help in building the project timeline by specifying the order in which activities should be executed. Relationships between activities help project managers understand the sequence of tasks, determine the project’s critical path, and analyze potential delays.
There are four types of activity relationships (or dependencies) in Primavera P6, each serving a different purpose in defining the sequence of activities.
1. Finish-to-Start (FS)
Description: This is the most commonly used relationship type. In a Finish-to-Start relationship, the successor activity cannot start until the predecessor activity finishes.
Example:
- Predecessor Activity: Build the foundation of a building.
- Successor Activity: Start constructing the walls.
In this case, the walls cannot be constructed until the foundation is complete.
Real-Life Scenario:
- In a software development project, coding (successor) cannot begin until the design phase (predecessor) is finished. This ensures that the coders have the necessary requirements and specifications before they begin working.
2. Start-to-Start (SS)
Description: In a Start-to-Start relationship, the successor activity cannot start until the predecessor activity starts. Both activities can continue in parallel after they start.
Example:
- Predecessor Activity: Start excavating the site for a building.
- Successor Activity: Begin laying the foundation.
Once the excavation starts, you can start laying the foundation. Both activities happen simultaneously, but the laying of the foundation cannot begin until the excavation starts.
Real-Life Scenario:
- In road construction, once the grading of the road begins (predecessor), paving the surface (successor) can start as the grader works in different areas of the site. This saves time since both activities progress together.
3. Finish-to-Finish (FF)
Description: In a Finish-to-Finish relationship, the successor activity cannot finish until the predecessor activity finishes. Both activities can be ongoing simultaneously, but the successor cannot complete until the predecessor is done.
Example:
- Predecessor Activity: Complete wiring installation.
- Successor Activity: Complete electrical system testing.
The testing of the electrical system can continue in parallel with wiring installation, but testing cannot be completed until all wiring installation is finished.
Real-Life Scenario:
- In large-scale events such as concerts, the setup of the sound system (predecessor) and the sound check (successor) may occur simultaneously, but the sound check cannot be completed until the sound system setup is fully finished.
4. Start-to-Finish (SF)
Description: The Start-to-Finish relationship is the least commonly used. In this relationship, the successor activity cannot finish until the predecessor activity starts. It is typically used in scenarios where one activity must start before another can end.
Example:
- Predecessor Activity: Start the night shift of security.
- Successor Activity: End the day shift of security.
The night shift must start before the day shift can finish, ensuring there is no gap in security coverage.
Real-Life Scenario:
- In hospital management, the night nursing staff (predecessor) must be present before the day nursing staff (successor) can leave. This ensures continuous patient care.
Additional Concepts Related to Activity Relationships
1. Lag and Lead Time
- Lag Time: A delay introduced between two activities. For instance, if there is a 3-day lag between a Finish-to-Start relationship, the successor activity will only start 3 days after the predecessor has finished. Example: After pouring concrete (predecessor), there might be a 7-day lag before the curing process (successor) starts.
- Lead Time: A lead (or negative lag) allows the successor activity to start before the predecessor activity finishes. Example: If painting the first floor can begin when 80% of the plastering is done, the project schedule can include a lead to reflect this overlap.
2. Critical Path
The Critical Path Method (CPM) uses activity relationships to determine the longest path of dependent activities from the start to the end of a project. The critical path determines the minimum project duration. Any delay in critical path activities will delay the entire project.
Conclusion
Understanding and using the different types of activity relationships in Primavera P6 is essential for accurate project scheduling, resource allocation, and risk management. These relationships (Finish-to-Start, Start-to-Start, Finish-to-Finish, and Start-to-Finish) provide the framework for how activities interact, helping project managers define the logical flow of tasks, identify the critical path, and analyze potential scheduling delays.
