Dependency Determination and Types in CPM Scheduling
5 minutes
5 Questions
In Critical Path Method (CPM) scheduling, determining task dependencies is fundamental to constructing an accurate project schedule. Dependencies define how tasks relate to one another and establish the sequence of activities. There are four primary types of dependencies in CPM scheduling:
1. **Fi…In Critical Path Method (CPM) scheduling, determining task dependencies is fundamental to constructing an accurate project schedule. Dependencies define how tasks relate to one another and establish the sequence of activities. There are four primary types of dependencies in CPM scheduling:
1. **Finish-to-Start (FS)**: The most common dependency where a successor task cannot begin until its predecessor task has finished. For example, painting cannot start until surface preparation is complete.
2. **Start-to-Start (SS)**: A task cannot start until its predecessor has started. This allows two activities to begin simultaneously or with a delay defined by a lag. For instance, programming can start once design has started.
3. **Finish-to-Finish (FF)**: A task cannot finish until its predecessor has finished. This is useful when two activities must conclude together. For example, quality assurance must finish when testing concludes.
4. **Start-to-Finish (SF)**: The least common dependency where a task cannot finish until its predecessor has started. This might occur in scenarios like shift changes where the new shift must begin before the previous one can end.
Understanding these dependencies is crucial for accurate schedule development. It ensures that tasks are sequenced logically and resources are allocated efficiently. Dependencies can be further classified as **mandatory**, **discretionary**, or **external**:
- **Mandatory Dependencies**: Inherent in the nature of the work, such as legal or physical constraints. For example, concrete must cure before formwork is removed.
- **Discretionary Dependencies**: Defined by the project team based on best practices or preferences. For instance, choosing to complete all design work before starting construction, even if some construction activities could technically begin earlier.
- **External Dependencies**: Involve relationships between project tasks and external events or activities outside the project's control, such as regulatory approvals or delivery of materials from a supplier.
Proper dependency determination helps identify the critical path accurately, assess project risks, and develop realistic schedules. It allows project managers to anticipate potential delays and implement mitigation strategies. Misidentifying dependencies can lead to scheduling conflicts, resource overallocation, and inaccurate timelines, adversely affecting project outcomes.
Comprehensive Guide to Dependency Types in CPM Scheduling
Why Dependency Types are Important in Project Management
Understanding dependency types in Critical Path Method (CPM) scheduling is crucial for project success because:
• Dependencies define the logical relationships between activities • They determine the sequence of work and overall project timeline • Proper dependency identification prevents resource conflicts and schedule delays • They form the foundation for critical path analysis and schedule optimization • PMI-SP certification exams heavily test this knowledge area
What are Dependencies in CPM Scheduling?
Dependencies in CPM scheduling are logical relationships that define how activities relate to each other in terms of sequence and timing. They answer the question: "What activities must finish or start before others can begin or end?" The four standard dependency types (also called logical relationships) are:
1. Finish-to-Start (FS): The most common type where a successor activity cannot start until a predecessor activity finishes. Example: Concrete must cure (finish) before framing can begin (start).
2. Start-to-Start (SS): A successor activity cannot start until a predecessor activity starts. Example: Quality inspection (successor) can begin once production (predecessor) starts.
3. Finish-to-Finish (FF): A successor activity cannot finish until a predecessor activity finishes. Example: System testing cannot finish until debugging is completed.
4. Start-to-Finish (SF): A successor activity cannot finish until a predecessor activity starts. Example: Night shift security guard cannot end shift until day shift guard starts.
How Dependency Determination Works in Practice
Determining appropriate dependencies involves:
1. Activity Definition: Clearly defining project activities 2. Relationship Analysis: Analyzing which activities must precede others 3. Dependency Type Selection: Choosing the appropriate relationship type 4. Lead/Lag Consideration: Adding time modifiers when needed: - Lead time: Allows partial overlap (acceleration) of activities - Lag time: Enforces waiting periods between activities 5. Network Diagram Creation: Graphically representing all relationships 6. Schedule Calculation: Computing early/late start/finish dates
Categories of Dependencies
Dependencies can be classified as:
• Mandatory Dependencies (Hard Logic): Physically impossible to ignore Example: Paint must dry before furniture is placed
• Discretionary Dependencies (Preferred Logic): Based on best practices or preferences Example: Completing all plumbing before electrical work although partial overlap is possible
• External Dependencies: Relationships between project and non-project activities Example: Waiting for regulatory approval before proceeding
• Internal Dependencies: Relationships between activities within the project team's control Example: Testing must follow development
Exam Tips: Answering Questions on Dependency Determination and Types
1. Understand the terminology fully • Know all four dependency types and their abbreviations (FS, SS, FF, SF) • Recognize that FS is the default and most common type • Remember SF is the least common type
2. Practice identifying the correct dependency type • Read scenarios carefully to determine logical relationships • Ask: "What must happen first and how does it relate to what follows?" 3. Master dependency categorization • Differentiate between mandatory, discretionary, external, and internal dependencies • Learn to identify the appropriate category from scenario descriptions
4. Watch for lead and lag applications • Understand how lead (-) and lag (+) modify dependencies • Example: FS+10 means a 10-day lag after predecessor finishes
5. Apply critical thinking to complex networks • For complex questions, sketch a simple network diagram • Trace paths to identify critical activities and relationships
6. Connect dependencies to critical path analysis • Understand how changing dependencies affects the critical path • Recognize that lengthening critical path activities extends the project duration
7. Be aware of common exam traps • Question may describe a relationship but ask for the formal dependency type • Multiple relationships might exist between activities; identify the primary one • Some questions may include unnecessary information
The PMI-SP exam often presents scenario-based questions that require applying dependency knowledge to solve scheduling problems. Focus on understanding the concepts rather than memorizing definitions, and practice analyzing project scenarios to determine appropriate dependencies.
PMI-SP - Dependency Determination and Types in CPM Scheduling Example Questions
Test your knowledge of Dependency Determination and Types in CPM Scheduling
Question 1
According to PMI scheduling standards, which dependency type best describes when an activity's completion allows the start of its successor, but the successor can also begin earlier with an overlap?
Question 2
In CPM scheduling, what determines a Finish-to-Start (FS) mandatory dependency between two construction activities?
Question 3
In CPM scheduling, which type of dependency would best reflect a requirement where an activity must maintain a constant time interval of 5 days ahead of its successor throughout their durations?
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