Precedence Diagramming Method: The Four Logical Relationships

PMBOK v8 Definition

The precedence diagramming method (PDM) is a technique used for constructing a schedule model in which activities are represented by nodes and are graphically linked by one or more logical relationships to show the sequence in which the activities are to be performed. These logical relationships are dependencies between two activities or between an activity and a milestone, and include four types: finish-to-start (FS), finish-to-finish (FF), start-to-start (SS), and start-to-finish (SF).

Why It Matters for the Exam

This concept appears frequently in PMI exam questions on schedule management, particularly in questions testing your ability to interpret network diagrams and calculate schedule dates. Expect 2-4 questions on identifying relationship types, understanding their sequence implications, and recognizing which relationship is most commonly used versus rarely used in practice.

Key Points to Remember (for the exam)

• Most Common Relationship: Finish-to-start (FS) is the most commonly used type of precedence relationship in the precedence diagramming method.
• Rarely Used Relationship: Start-to-finish (SF) is very rarely used but is included to present a complete list of relationship types.
• Four Relationship Types: FS (Finish-to-Start), FF (Finish-to-Finish), SS (Start-to-Start), SF (Start-to-Finish)
• Multiple Relationships: Two activities can have two logical relationships at the same time (for example, SS and FF). However, multiple relationships among the same activities are not recommended.
• Decision Rule: When multiple relationships exist between the same activities, a decision must be made to select the relationship with the highest impact.
• Closed Loops: Closed loops are not recommended in logical relationships.
• Predecessor vs. Successor: A predecessor activity logically comes before a dependent activity; a successor activity is a dependent activity that logically comes after another activity.

Typical PMI Exam Example

A project manager is developing the project schedule network diagram. Activity B cannot start until Activity A has finished. This is an example of which type of logical relationship? Answer: Finish-to-start (FS). In this relationship, the successor activity (B) cannot start until the predecessor activity (A) has finished. For example, installing the operating system on a PC (successor) cannot start until the PC hardware is assembled (predecessor).

PMI Exam Traps

• Trap: Confusing FF with FS when both involve "finish"

  • Reality: FS means the successor cannot start until predecessor finishes; FF means the successor cannot finish until predecessor finishes

• Trap: Thinking SF is commonly used or recommended

  • Reality: SF is very rarely used; FS is the most commonly used relationship type

• Trap: Assuming two activities can only have one relationship type

  • Reality: Two activities can have two logical relationships at the same time (e.g., SS and FF), though this is not recommended

• Trap: Confusing predecessor (comes before) with successor (comes after)

  • Reality: Predecessor activity logically comes before a dependent activity; successor activity logically comes after another activity

Important PMI Connections

Quick Review Questions

1. In the precedence diagramming method, which logical relationship type is most commonly used?

2. An activity that logically comes before a dependent activity in a schedule is called what?

3. Two activities have both an SS and an FF relationship. According to PMBOK v8, what should the project manager do?

4. Which relationship type is very rarely used but is included to present a complete list of relationship types?

5. What is the key difference between a finish-to-finish (FF) relationship and a finish-to-start (FS) relationship?

PMBOK v8 Reference

Section 5.2.2.3 - Precedence Diagramming Method (Figure 5-13: Precedence Diagramming Method Relationship Types)