Dependency Determination and Types in CPM

Dependency Determination and Types in Critical Path Method (CPM)

Why Dependency Determination is Important in CPM

Dependency determination is a fundamental component of the Critical Path Method because it establishes the logical relationships between activities in a project schedule. These relationships determine the sequence of work and ultimately affect the critical path calculation. Without properly identified dependencies, a project schedule becomes unreliable, leading to:

• Unrealistic timelines
• Resource conflicts
• Inability to identify the critical path accurately
• Ineffective schedule compression attempts
• Poor project control and monitoring

What are Dependencies in CPM?

Dependencies in CPM represent the logical relationships between activities that determine the sequence in which project work must be performed. These relationships answer the question: "What activities must finish before others can start?"
A dependency consists of a predecessor activity (which comes first) and a successor activity (which follows). The relationship between these activities is defined by one of four dependency types.

The Four Types of Dependencies in CPM

1. Finish-to-Start (FS) - The most common dependency type where a successor activity cannot start until its predecessor activity has finished.
Example: Concrete must cure (finish) before framing can begin (start).

2. Start-to-Start (SS) - A successor activity cannot start until its predecessor activity has started.
Example: Quality inspection (successor) can begin once production (predecessor) has started.

3. Finish-to-Finish (FF) - A successor activity cannot finish until its predecessor activity has finished.
Example: System testing cannot finish until all coding work has finished.

4. Start-to-Finish (SF) - A successor activity cannot finish until its predecessor activity has started.
Example: The current shift cannot end until the next shift begins.

How Dependencies Affect the Critical Path

The critical path is calculated by analyzing all activity sequences through the network diagram. Dependencies determine:

• Which activities can be performed in parallel
• Which activities must be performed sequentially
• The early start, early finish, late start, and late finish dates for each activity
• The total float or slack for each activity
• The overall project duration

Categories of Dependencies

Mandatory Dependencies (Hard Logic)
• Physical limitations that cannot be changed
• Example: You must pour a foundation before building walls

Discretionary Dependencies (Soft Logic)
• Based on best practices or preferences
• Example: Choosing to complete design before starting procurement

External Dependencies
• Dependencies on elements outside the project team's control
• Example: Waiting for regulatory approval before proceeding

Internal Dependencies
• Dependencies between activities within the project team's control
• Example: Designing before building

Dependency Determination Process

1. Identify activities - Break down the project into manageable activities
2. Sequence activities - Determine the logical order
3. Select dependency type - Choose the appropriate relationship (FS, SS, FF, SF)
4. Document dependencies - Create a network diagram or precedence diagram
5. Review for completeness - Ensure all activities have at least one predecessor and successor (except for the first and last activities)

Lead and Lag Time in Dependencies

Lead Time allows a successor activity to start earlier than it normally would according to the dependency type. It's an acceleration or overlap of activities.
Example: In a FS relationship with a 5-day lead, the successor can start 5 days before the predecessor finishes.

Lag Time adds a delay between activities. It extends the waiting time between the predecessor and successor.
Example: In a FS relationship with a 3-day lag, the successor must wait 3 days after the predecessor finishes before it can start.

Common Pitfalls in Dependency Determination

• Creating circular dependencies where activities depend on each other
• Overusing discretionary dependencies, creating unnecessary restrictions
• Using the wrong dependency type for the relationship
• Failing to account for external dependencies
• Creating too many dependencies, making the schedule rigid
• Not considering resource availability when determining dependencies

Exam Tips: Answering Questions on Dependency Determination and Types in CPM

1. Know the four dependency types cold - Be able to identify and explain FS, SS, FF, and SF relationships.

2. Understand the different categories - Be prepared to distinguish between mandatory, discretionary, external, and internal dependencies.

3. Practice network diagramming - Many exam questions involve interpreting or creating network diagrams.

4. Calculate with lead and lag - Know how to incorporate lead and lag times into schedule calculations.

5. Apply critical thinking - For scenario-based questions, think about the practical implications of dependencies in real project situations.

6. Watch for relationship identification - Be ready to identify the correct dependency type from a description of two activities.

7. Understand dependency impacts - Know how changing a dependency affects the critical path and project duration.

8. Recognize improper dependencies - Be able to spot invalid dependencies like circular relationships.

9. Connect to other knowledge areas - Relate dependencies to risk management, resource management, and scope management.

10. Use process of elimination - For multiple-choice questions, eliminate answers that are completely illogical before selecting from remaining options.

Remember that dependency determination is the foundation of schedule development. On the exam, you'll need to demonstrate that you understand not just the technical aspects of dependencies, but also their practical application in creating realistic and manageable project schedules.