Precedence Diagramming Method (PDM) and Dependencies – A Complete Guide for PMP Exam Success
Why Is the Precedence Diagramming Method (PDM) and Understanding Dependencies Important?
The Precedence Diagramming Method (PDM) is the most widely used technique for constructing project schedule models. It forms the foundation of how project managers visualize, sequence, and manage the order in which project activities are performed. Understanding PDM and dependencies is critical because:
• It enables accurate schedule development and realistic timelines
• It helps identify the critical path and schedule flexibility
• It supports effective resource allocation and workload balancing
• It allows project managers to assess the impact of changes and delays
• It is a heavily tested topic on the PMP exam, appearing in questions about scheduling, planning, and predictive methodologies
Without a thorough understanding of PDM and dependencies, a project manager cannot create a reliable schedule or effectively manage project timelines.
What Is the Precedence Diagramming Method (PDM)?
The Precedence Diagramming Method (PDM) is a technique used to construct a schedule model in which activities are represented as nodes (boxes) and the logical relationships (dependencies) between them are represented as arrows connecting those nodes. PDM is also known as Activity-On-Node (AON).
PDM is the method used by virtually all modern project management software tools (such as Microsoft Project, Primavera, etc.) to create project network diagrams. It replaced the older Activity-On-Arrow (AOA) method because of its flexibility in representing all four types of logical relationships.
Key characteristics of PDM:
• Activities are placed in boxes (nodes)
• Arrows show the sequence and dependency between activities
• It supports all four types of logical relationships
• It allows the use of leads and lags
• It is the standard method referenced in the PMBOK® Guide
The Four Types of Logical Relationships (Dependencies) in PDM
PDM supports four types of logical relationships between a predecessor activity and a successor activity. Understanding each one is essential for the PMP exam.
1. Finish-to-Start (FS)
The successor activity cannot start until the predecessor activity finishes.
• This is the most commonly used relationship type
• Example: You cannot begin painting a wall (successor) until the plastering (predecessor) is complete
• Notation: Activity A must finish before Activity B can start
2. Start-to-Start (SS)
The successor activity cannot start until the predecessor activity starts.
• Both activities can run in parallel after the predecessor begins
• Example: Leveling concrete (successor) can begin once pouring concrete (predecessor) has started
• Notation: Activity B cannot start until Activity A starts
3. Finish-to-Finish (FF)
The successor activity cannot finish until the predecessor activity finishes.
• Both activities may run concurrently, but the successor cannot complete until the predecessor does
• Example: The editing of a document (successor) cannot finish until the writing of the document (predecessor) is finished
• Notation: Activity B cannot finish until Activity A finishes
4. Start-to-Finish (SF)
The successor activity cannot finish until the predecessor activity starts.
• This is the least commonly used relationship type
• Example: The old security system (successor) cannot be decommissioned (finished) until the new security system (predecessor) starts operating
• Notation: Activity B cannot finish until Activity A starts
• Exam Tip: SF is rarely used in practice but is frequently tested on the exam. Remember it as the rarest relationship.
Types of Dependencies
Beyond the four logical relationships, dependencies themselves are classified into categories based on their nature and origin. The PMP exam tests your knowledge of these categories:
1. Mandatory Dependencies (Hard Logic)
• These are inherent to the nature of the work and cannot be changed
• They are often based on physical limitations or contractual requirements
• Example: A foundation must be laid before building walls
• Also called: hard logic, hard dependencies
2. Discretionary Dependencies (Soft Logic)
• These are established based on best practices, preferences, or team experience
• They can be modified if needed (e.g., during schedule compression techniques like fast tracking)
• Example: A team prefers to complete all design work before starting any coding, even though some coding could begin earlier
• Also called: soft logic, preferred logic, preferential logic
• Exam Tip: Fast tracking primarily targets discretionary dependencies for modification
3. External Dependencies
• These involve a relationship between a project activity and a non-project activity
• They are outside the project team's control
• Example: A government permit must be obtained before construction can begin
• These dependencies introduce risk because the project team cannot influence them directly
4. Internal Dependencies
• These involve relationships between project activities that are within the project team's control
• Example: Testing cannot begin until coding is complete (both are internal project activities)
Important Note: Dependencies can be combinations. For example, a dependency can be both mandatory and external or discretionary and internal. The exam may test your ability to classify a dependency into multiple categories simultaneously.
Leads and Lags
Leads and lags are adjustments applied to logical relationships to refine the schedule:
Lead: The amount of time a successor activity can be advanced with respect to the predecessor. A lead accelerates the successor.
• Example: In an FS relationship with a 5-day lead, the successor can start 5 days before the predecessor finishes
• Leads are expressed as negative values (e.g., FS -5 days)
Lag: The amount of time a successor activity must be delayed with respect to the predecessor. A lag introduces a waiting period.
• Example: After pouring concrete (predecessor), you must wait 3 days for curing before beginning the next activity (successor). This is a 3-day lag.
• Lags are expressed as positive values (e.g., FS +3 days)
Exam Tip: Do not confuse lag with float (slack). Lag is a deliberate, planned delay built into the schedule. Float is the amount of time an activity can be delayed without affecting the project end date or a successor's early start.
How PDM Works in Practice
Here is the step-by-step process of using PDM to build a project schedule network diagram:
1. Identify all project activities from the Work Breakdown Structure (WBS) and activity list
2. Determine the logical relationships between activities (which must come before which, and what type of relationship: FS, SS, FF, SF)
3. Classify the dependencies as mandatory, discretionary, external, or internal
4. Apply leads and lags where appropriate
5. Draw the network diagram with activities as nodes and arrows showing dependencies
6. Use the network diagram as input for the Critical Path Method (CPM), schedule compression, and other scheduling techniques
The resulting network diagram provides a visual representation of the project schedule logic, enabling forward and backward pass calculations, identification of the critical path, and determination of float values for each activity.
Relationship Between PDM and Other Scheduling Concepts
• Critical Path Method (CPM): Uses the PDM network diagram to calculate the longest path through the network, determining the minimum project duration
• Fast Tracking: Involves performing activities in parallel that were originally planned in sequence — this primarily modifies discretionary dependencies
• Crashing: Adds resources to critical path activities to shorten duration — does not change dependencies
• Resource Optimization: May require adjustments to the network diagram based on resource availability
Common Exam Scenarios and How to Approach Them
Scenario 1: You are given a network diagram and asked to identify the relationship type between two activities.
• Focus on what triggers the successor: Does it depend on the predecessor's start or finish? Does the constraint apply to the successor's start or finish?
• Use the naming convention: The first word refers to the predecessor, the second word refers to the successor (e.g., Finish-to-Start means predecessor finishes, then successor starts)
Scenario 2: You are asked which dependency type can be modified during fast tracking.
• The answer is discretionary (soft logic) dependencies
• Mandatory dependencies cannot be changed
Scenario 3: You are asked about the least commonly used logical relationship.
• The answer is Start-to-Finish (SF)
Scenario 4: You are presented with a situation involving a waiting period between activities.
• This describes a lag
• Do not confuse it with float or lead
Scenario 5: An activity depends on a government approval that is outside the team's control.
• This is an external dependency
• It may also be mandatory if it is legally required
Exam Tips: Answering Questions on Precedence Diagramming Method (PDM) and Dependencies
1. Memorize the four relationship types and their definitions: FS (most common), SS, FF, SF (least common). Know what triggers the successor in each case.
2. Remember the naming convention: The first term always refers to the predecessor and the second term always refers to the successor. Finish-to-Start = Predecessor Finishes → Successor Starts.
3. Know that Finish-to-Start (FS) is the most commonly used and Start-to-Finish (SF) is the least commonly used relationship. The exam frequently asks about both.
4. Distinguish between mandatory and discretionary dependencies: Mandatory cannot be changed (hard logic); discretionary can be changed (soft logic). Fast tracking targets discretionary dependencies.
5. Understand external vs. internal dependencies: External dependencies are outside the team's control and introduce risk. Internal dependencies are within the team's control.
6. Remember that dependencies can be combinations: A dependency can be both mandatory and external, or discretionary and internal. Be prepared for questions that ask you to classify a dependency into two categories.
7. Do not confuse lead with lag: Lead accelerates (negative time); lag delays (positive time). Both are applied to logical relationships.
8. Do not confuse lag with float: Lag is a planned, intentional waiting period built into the schedule. Float is the flexibility an activity has before it delays the project or a successor.
9. PDM = Activity-On-Node (AON): If the exam mentions AON, it is referring to PDM. Activities are nodes (boxes); arrows are relationships.
10. When reading scenario questions, identify keywords: "Cannot start until" often indicates FS or SS. "Cannot finish until" often indicates FF or SF. "Waiting period" indicates lag. "Overlap" or "advance" indicates lead.
11. Practice drawing small network diagrams: On exam day, use your scratch paper to sketch quick network diagrams for scheduling questions. Visual representation makes it much easier to trace relationships and calculate paths.
12. Connect PDM to the bigger picture: PDM feeds into Critical Path Method, schedule compression (fast tracking and crashing), and risk analysis (Monte Carlo simulation). Understand how dependencies affect the critical path and total float.
13. For PMBOK 7/8 and the current PMP exam: While the exam is less process-focused, PDM and dependencies remain core scheduling concepts tested across predictive, hybrid, and even agile contexts (where dependencies between iterations or teams matter).
14. Watch for trick answers: The exam may present a scenario that sounds like one relationship type but is actually another. Always carefully analyze what specific condition (start or finish) of which activity (predecessor or successor) is being described.
By mastering these concepts and practicing with scenario-based questions, you will be well-prepared to tackle any PDM and dependency question on the PMP exam with confidence.
