Project Scheduling: Precedence Relationships

Introduction to Precedence Relationships

Precedence relationships define the logical sequence in which project activities must be performed. These dependencies are fundamental to creating accurate project schedules and identifying the critical path.

Types of Precedence Relationships

1. Finish-to-Start (FS)

The most common dependency type where Activity B cannot start until Activity A finishes.

Example: You can't start painting a wall (B) until the plastering is complete (A).

[Activity A] → [Activity B]

FS with no lag: A must finish before B can start

2. Start-to-Start (SS)

Activity B cannot start until Activity A starts.

Example: Quality testing (B) can start once coding (A) begins, but doesn't need to wait for coding to finish.

[Activity A] → [Activity B] (start points aligned)

SS with 2-day lag: B can start 2 days after A starts

3. Finish-to-Finish (FF)

Activity B cannot finish until Activity A finishes.

Example: Documentation (B) can't be completed until software development (A) is finished.

[Activity A] → [Activity B] (end points aligned)

FF with 1-day lag: B can finish 1 day after A finishes

4. Start-to-Finish (SF)

The least common type where Activity B cannot finish until Activity A starts.

Example: Night security shift (B) can't end until morning shift (A) begins.

[Activity A] → [Activity B] (A start aligns with B finish)

SF with 1-hour lag: B can finish 1 hour after A starts

Precedence Diagram Method (PDM)

The Precedence Diagram Method is a technique for constructing a project schedule network diagram that uses boxes/nodes to represent activities and connects them with arrows to show dependencies.

Sample PDM Diagram

[Activity A (FS)] → [Activity B (SS+2)] → [Activity C]

[Activity A] → [Activity D (FF-1)]

Key Components:

Nodes: Represent activities with duration estimates
Arrows: Show dependencies between activities
Lag/Lead: Time gaps or overlaps between dependent activities

Practical Example

Website Development Project

Activity	Description	Predecessor	Dependency Type	Lag
A	Requirements Gathering	-	-	-
B	UI Design	A	FS	0
C	Backend Development	A	FS	2 days
D	Frontend Development	B	SS	3 days
E	Testing	C, D	FF	1 day

Best Practices

Use Finish-to-Start relationships as your default unless another type better represents the real-world constraint
Document all dependencies clearly in your project management software
Review dependencies with your team to ensure they reflect actual workflow constraints
Consider both mandatory (hard logic) and discretionary (soft logic) dependencies
Use lags and leads carefully to model realistic timing between activities

Knowledge Check

1. What dependency type would you use to model this situation: "The software demo can begin once development has started, but doesn't need to wait for development to finish"?

2. If Activity X must finish at least 2 days before Activity Y can start, how would you model this in a precedence diagram?

3. Why might you choose a Start-to-Finish relationship over other types?