Precedence Diagramming Method (PDM) for the PMP Exam Explained

Precedence Diagramming Method: Building the Network That Drives Your Schedule

The Precedence Diagramming Method is the technique used to construct the project schedule network diagram. On the PMP exam, PDM appears in questions about activity sequencing, dependency types, and network diagram interpretation. It is the only method currently recognized by the PMBOK Guide for creating schedule network diagrams, making it unavoidable on the exam.

The Four Dependency Types in PDM

PDM defines four logical relationships between activities. You must memorize these and recognize them in scenario questions:

Finish-to-Start (FS)

The successor cannot start until the predecessor finishes. This is the most common relationship and the default in most scheduling software. Example: "Pouring concrete (predecessor) must finish before framing walls (successor) can start."

Finish-to-Finish (FF)

The successor cannot finish until the predecessor finishes. Both activities can be in progress simultaneously, but the successor's completion depends on the predecessor's completion. Example: "Writing the document (predecessor) must finish before editing the document (successor) can finish."

Start-to-Start (SS)

The successor cannot start until the predecessor starts. The activities can overlap significantly. Example: "Leveling the foundation (predecessor) must start before pouring concrete (successor) can start."

Start-to-Finish (SF)

The successor cannot finish until the predecessor starts. This is the rarest relationship and often confuses PMP candidates. Example: "The new security system (predecessor) must start before the old security system (successor) can finish (be decommissioned)." The PMP exam almost always includes one question testing whether you understand SF.

Dependency Categories

Beyond the four logical relationships, PDM dependencies are categorized by their nature:

• Mandatory (hard logic): Inherent in the nature of the work. You cannot install plumbing before the walls exist.
• Discretionary (preferred logic): Established based on best practices or team preference. You could test module A before module B, but the team prefers a specific order.
• External: Involves a relationship between project activities and non-project activities. Waiting for a government permit is an external dependency.
• Internal: A relationship between project activities that the project team can control.

On the PMP exam, questions may ask which dependencies can be adjusted during schedule compression. The answer is discretionary dependencies — mandatory and external dependencies generally cannot be changed.

Constructing a Network Diagram with PDM

To build a network diagram using PDM:

1. List all project activities (from the activity list, which is derived from the WBS).
2. Identify the logical relationships between each pair of activities.
3. Draw nodes for each activity (Activity-on-Node format).
4. Connect nodes with arrows representing the dependency type (FS, FF, SS, or SF).
5. Add leads and lags where applicable (leads accelerate the relationship; lags add waiting time).

PDM Exam Strategies

When facing PDM questions on the PMP exam:

• Default to Finish-to-Start if the relationship type is not specified — it is by far the most common.
• If a question describes activities happening simultaneously, look for SS or FF relationships.
• Remember that SF is counterintuitive — the successor finishes when the predecessor starts. Read carefully.
• Leads and lags modify these relationships. A lead on an FS relationship means the successor can start before the predecessor finishes.

Test your ability to read and construct network diagrams with our interactive practice questions designed specifically for the Process domain.