Project Schedule Network Diagram: Logical Relationships and Dependencies
PMBOK v8 Definition
The project schedule network diagram is a graphical representation of the logical relationships among the project schedule activities. These logical relationships may also be referred to as dependencies. The diagram can be produced manually or by using project management software, and may include full project details or one or more summary activities with an accompanying narrative describing the basic approach used to sequence the activities.
Why It Matters for the Exam
The project schedule network diagram appears frequently on the PMI exam in questions about schedule development, sequencing activities, and understanding dependency types. Exam questions typically test your ability to identify correct logical relationships, recognize the purpose of the diagram in creating a realistic schedule, and distinguish it from other scheduling artifacts like Gantt charts or milestone charts.
Key Points to Remember (for the exam)
- Main Purpose: Graphical representation of logical relationships (dependencies) among project schedule activities
- Four Logical Relationships Tested: Finish-to-Start (FS), Finish-to-Finish (FF), Start-to-Start (SS), Start-to-Finish (SF)
- Key Constraint: Every work package and activity, except for the first and last, should be connected to at least one predecessor and at least one successor activity with an appropriate logical relationship
- Critical Concept: Logical relationships should be designed to create a realistic project schedule—lead or lag time may be necessary
- Common Confusion: The diagram itself is NOT the schedule; it represents the sequence and dependencies, while the schedule includes dates, durations, and resources
- Key Benefit: Defines the logical sequence of work to obtain the greatest efficiency, given all project constraints
- Production Method: Can be created manually or using project management software
Typical PMI Exam Example
A project manager is developing the project schedule and needs to show the sequence of activities and their dependencies. Which document should the project manager create to graphically display the logical relationships among activities?
Answer: Project schedule network diagram
PMI Exam Traps
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Trap: Confusing the project schedule network diagram with the project schedule itself
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Reality: The diagram shows logical relationships and sequence; the schedule includes dates, durations, and resource assignments
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Trap: Thinking lead and lag are types of logical relationships
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Reality: Lead and lag are adjustments applied TO logical relationships (e.g., FS with 5 days lag), not separate relationship types
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Trap: Assuming all activities must have both predecessor and successor
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Reality: The first and last activities are exceptions—they may have only a successor or only a predecessor
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Trap: Confusing "logical relationship" with "dependency"
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Reality: PMBOK v8 treats these as the same concept—logical relationships may also be referred to as dependencies
Important PMI Connections
| Related Concept | Relationship Type | Exam Attention Point |
|---|---|---|
| Schedule Forecast | Complementary | Forecasts update the schedule based on progress; network diagrams show the planned sequence |
| Actual Duration | Input to Schedule Control | Actual duration is measured against the logical sequence defined in the diagram |
| Risk Management Activities | Timing Input | Risk activities must be included in the project schedule and shown in network diagrams |
| Make-or-Buy Analysis | Predecessor Process | Before sequencing, decisions about which activities to perform internally vs. purchase affect the diagram |
Quick Review Questions
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What are the four types of logical relationships in a project schedule network diagram?
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Which activities in a project schedule network diagram are NOT required to have both a predecessor and a successor?
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What is the key benefit of defining logical relationships among schedule activities?
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When might it be necessary to use lead or lag time between activities in a network diagram?
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What is the difference between a project schedule network diagram and the project schedule itself?
PMBOK v8 Reference
Section 4 - Project Schedule Network Diagram (also referenced in Section 2.3)