Understanding Dependencies (Finish-to-Start, Start-to-Start, etc.)

Lecture Notes: Understanding Dependencies (Finish-to-Start, Start-to-Start, etc.)

Introduction

Once project activities have been identified, the next step in scheduling is to determine how these activities relate to each other — their dependencies.
Dependencies define the logical order in which tasks must occur, ensuring work flows smoothly without conflicts or idle time.
In essence, they show which tasks must wait for others and which can run in parallel.

 What Are Activity Dependencies?

An activity dependency (or logical relationship) is the connection between two or more tasks that determines the sequence in which they are performed.

Each dependency defines:

• When an activity can start or finish, relative to another activity.
• How changes in one activity’s timing affect another.

Example:
You can’t paint a wall until the wall has been built — that’s a dependency.

Types of Dependencies

Dependencies are grouped based on why the relationship exists.

 Logical Relationship Types (Dependency Relationships)

There are four standard logical relationships used in project scheduling tools like MS Project or Primavera:

Examples Explained

1. Finish-to-Start (FS):

• Most common relationship.
• Activity B cannot begin until Activity A is complete.
  Example:
  A: Design website → B: Develop website.
  (Development begins only after design is complete.)

2. Start-to-Start (SS):

• Activities start at roughly the same time but may progress independently.
  Example:
  A: Begin writing content → B: Begin designing layout.
  (Both can start together once initial inputs are ready.)

3. Finish-to-Finish (FF):

• Two activities must finish at the same time.
  Example:
  A: Testing system → B: Documenting test results.
  (Documentation can continue as testing happens but ends when testing ends.)

4. Start-to-Finish (SF):

• The finish of one activity depends on the start of another.
  (Rare and often used for scheduling shift changes or system transitions.)
  Example:
  A: Start night shift → B: Finish day shift.
  (The day shift ends only when the night shift begins.)

Dependency Representation in a Schedule

Dependencies are usually shown through network diagrams or Gantt charts.

• Network Diagram: Activities are shown as nodes connected by arrows that represent dependencies.
• Gantt Chart: Bars show task durations and lines/arrows show the dependency relationships between tasks.

Example (simplified):

A (Build wall) → B (Paint wall) → C (Inspect)

 Leads and Lags

Dependencies can include time adjustments called leads and lags.

Leads and lags fine-tune the schedule for realistic timing.

 Importance of Understanding Dependencies

• Builds a logical project schedule.
• Identifies critical paths and potential bottlenecks.
• Helps manage resource allocation and avoid overloading.
• Improves forecasting and risk analysis (what happens if one task slips?).
• Ensures smooth workflow across teams and deliverables.

Common Mistakes

• Missing dependencies: Forgetting links between related tasks can cause unrealistic timelines.
• Incorrect logic: Forcing parallel work where sequential work is required leads to rework.
• Too many constraints: Over-constraining the schedule reduces flexibility.
• Ignoring external factors: Missing client approvals or vendor timelines.

 Summary Table

Conclusion

Understanding dependencies allows project managers to map how work truly flows — not just what needs to be done, but in what order and under what conditions.
Mastering these relationships helps build a realistic, flexible schedule that anticipates both collaboration and constraint.
In scheduling, logic is power — and dependencies are that logic made visible.