Logical Relationships in Scheduling: Predecessor, Successor, Finish-to-Start

PMBOK v8 Definition

A predecessor activity is an activity that logically comes before a dependent activity in a schedule. A successor activity is a dependent activity that logically comes after another activity in a schedule. The Finish-to-Start (FS) relationship is a logical relationship in which a successor activity cannot start until a predecessor activity has finished. This is the most commonly used type of precedence relationship in the precedence diagramming method.

Why It Matters for the Exam

The PMI exam frequently tests your ability to identify and apply the four types of logical relationships, particularly Finish-to-Start, as it appears in 60-70% of schedule network diagram questions. Expect scenario-based questions where you must determine the correct relationship type based on activity descriptions, or identify which relationship is being used in a given project situation.

Key Points to Remember (for the exam)

Four Relationship Types: FS (Finish-to-Start), FF (Finish-to-Finish), SS (Start-to-Start), SF (Start-to-Finish)
Most Common: FS is the most commonly used type of precedence relationship
Least Common: SF (Start-to-Finish) is very rarely used
Multiple Relationships: Two activities can have two logical relationships at the same time (e.g., SS and FF) but this is not recommended
Closed Loops: Not recommended in logical relationships
Every Activity: Except for the first and last, should be connected to at least one predecessor and one successor
Key Benefit: Defines the logical sequence of work to obtain the greatest efficiency

Typical PMI Exam Example

A project involves installing an operating system on a new PC. The PC hardware must be fully assembled before the operating system installation can begin. Which logical relationship describes this situation? Answer: Finish-to-Start (FS) – the successor (OS installation) cannot start until the predecessor (hardware assembly) has finished.

PMI Exam Traps

Trap: Confusing FS with SS because both involve "starting" an activity Reality: FS requires the predecessor to FINISH before the successor can START; SS requires both to START together
Trap: Thinking SF is the same as FS but reversed Reality: SF means the predecessor cannot FINISH until the successor has STARTED – a rare but distinct relationship
Trap: Assuming all activities must have only one relationship type Reality: Two activities can have two relationships simultaneously (e.g., SS and FF), though this is not recommended
Trap: Forgetting that the first and last activities are exceptions Reality: Every work package and activity, except for the first and last, should be connected to at least one predecessor and one successor

Important PMI Connections

Related Concept	Relationship Type	Exam Attention Point
Precedence Diagramming Method (PDM)	Contains all four relationship types	PDM is the tool that uses FS, FF, SS, SF relationships
Lead and Lag Time	Applied to logical relationships	May be necessary to support a realistic project schedule
Schedule Network Diagram	Visual representation of relationships	FS is the most common relationship shown in network diagrams
Critical Path Method	Uses relationship types for calculation	FS relationships directly affect critical path calculations

Quick Review Questions

In which logical relationship does a successor activity cannot finish until a predecessor activity has finished?
A new accounts payable system must start before the old system can be shut down. Which relationship type is this?
What is the key benefit of determining dependencies and relationships among schedule elements?
True or False: Closed loops are recommended in logical relationships to improve schedule efficiency.
Which relationship type is the most commonly used in the precedence diagramming method?

PMBOK v8 Reference

Section 5 – Tools and Techniques, Precedence Diagramming Method (Figure 5-13)