Logical Relationships and Network Diagrams

Lecture Notes: Logical Relationships and Network Diagrams

 Introduction

After defining project activities, the next step in schedule development is to determine how these activities are related — the logical relationships — and to represent them visually using network diagrams.
Together, they form the blueprint of project sequencing, showing how work will flow from start to finish.

 What Are Logical Relationships?

A logical relationship describes the dependency or connection between two project activities that dictates the order in which they must be performed.

These relationships ensure the project schedule is realistic, coordinated, and aligned with how the work must actually occur.

Example:
You can’t test a system before it’s built — that’s a logical dependency.

Types of Logical Relationships

There are four primary types of logical relationships used in project scheduling tools (like MS Project or Primavera):

These relationships control the sequence, timing, and overlaps between tasks.

 Importance of Logical Relationships

• Defines the work sequence and project flow.
• Identifies dependencies that could cause delays.
• Helps in determining the critical path — the longest sequence of dependent tasks that defines the project’s minimum duration.
• Enables accurate forecasting of time, cost, and resources.
• Facilitates what-if analysis and rescheduling when plans change.

Understanding Leads and Lags

In real-world projects, dependencies are not always immediate. You can adjust them using leads and lags.

Leads and lags make the network more realistic by capturing true timing relationships.

Network Diagrams: Definition

A Network Diagram is a visual representation of project activities and their logical relationships.
It’s also known as a Project Network, Precedence Diagram, or Activity-on-Node (AON) diagram.

It shows:

• Activities as nodes or boxes.
• Dependencies as arrows connecting the nodes.

Types of Network Diagrams

Example (Activity-on-Node):

[A] Design → [B] Develop → [C] Test → [D] Deploy

Components of a Network Diagram

Steps in Creating a Network Diagram

1. List all project activities.
   • Derived from the WBS and activity list.
2. Identify dependencies.
   • Determine which tasks depend on others (FS, SS, FF, SF).
3. Sequence the activities logically.
   • Place them in order of execution.
4. Draw the diagram.
   • Represent each activity as a node, with arrows showing relationships.
5. Review and validate.
   • Ensure no missing or circular dependencies exist.

Example: Building a Website

Network Flow (AON):

[A] → [B] → [C] → [D] → [E]

This simple chain shows a pure Finish-to-Start sequence — each task begins only after the previous one ends.

Benefits of Network Diagrams

• Clarifies project logic — easy to see which tasks depend on others.
• Supports schedule development and analysis (e.g., critical path).
• Enhances communication among stakeholders.
• Aids in risk management by identifying where delays may occur.
• Improves control — visualizing progress and impact of changes.

 Common Mistakes

• Missing dependencies between activities.
• Creating circular logic (where tasks loop back).
• Overcomplicating the diagram with unnecessary detail.
• Ignoring external or discretionary dependencies.

Summary Table

Conclusion

Logical relationships define how work moves from one task to another, while network diagrams make that flow visible.
Together, they transform a simple task list into a structured, time-aware plan.
A well-built network diagram doesn’t just show when things happen — it shows why they happen in that order, which is the foundation of effective project scheduling and control.