Precedence Diagramming Method (PDM) and Dependencies in Project Schedule Management

Precedence Diagramming Method (PDM) and Dependencies in Project Schedule Management

Description
The Precedence Diagramming Method (PDM) is a technique used for creating project schedule network diagrams. It uses nodes (boxes) to represent activities and arrows to represent logical relationships (dependencies) between activities. Understanding PDM and the four types of dependency relationships is fundamental to building an accurate project schedule, which directly impacts the identification of the critical path and the project's total duration.

Knowledge Point Explanation

Step 1: Understanding the Basic Components of PDM

The core idea of PDM is: the start or finish of one activity depends on the start or finish of another activity. It consists of two basic elements:

  1. Node: Represents a specific project activity (or task). The node typically contains information such as the activity description, duration, earliest/latest start/finish times.
  2. Arrow: Represents the logical relationship or dependency between activities. The direction of the arrow indicates the direction of the dependency. For example, an arrow from Activity A to Activity B means Activity A is the predecessor to Activity B, and Activity B is the successor to Activity A.

Step 2: Mastering the Four Basic Types of Dependencies

In PDM, there are four types of dependency relationships between activities. This is the core of this knowledge point.

  1. Finish-to-Start (FS)

    • Description: The start of the successor activity depends on the finish of the predecessor activity. This is the most common and frequently used dependency.
    • Formula: B(start) depends on A(finish)
    • Example: "Painting" (Activity B) can only start after "Wall construction" (Activity A) is finished.
    • Diagram: [Activity A] --FS--> [Activity B]

  2. Finish-to-Finish (FF)

    • Description: The finish of the successor activity depends on the finish of the predecessor activity. This means the two activities can proceed concurrently, but the successor cannot finish before the predecessor finishes.
    • Formula: B(finish) depends on A(finish)
    • Example: "Writing the user manual" (Activity B) can be done at any time before "Software development" (Activity A) finishes, but the completion of "Writing the user manual" must wait until after "Software development" is finished (to ensure the manual content is complete).
    • Diagram: [Activity A] --FF--> [Activity B]

  3. Start-to-Start (SS)

    • Description: The start of the successor activity depends on the start of the predecessor activity. This means the successor can start after the predecessor starts, but they do not necessarily have to start simultaneously.
    • Formula: B(start) depends on A(start)
    • Example: After "Pouring concrete" (Activity A) starts, "Leveling the concrete" (Activity B) can begin. However, leveling does not need to start immediately at the beginning of pouring; it can start a bit later.
    • Diagram: [Activity A] --SS--> [Activity B]

  4. Start-to-Finish (SF)

    • Description: The finish of the successor activity depends on the start of the predecessor activity. This is the least common dependency, typically used only in specific scenarios.
    • Formula: B(finish) depends on A(start)
    • Example: The work of an "Old system maintenance team" (Activity B) must continue until the "New system go-live" (Activity A) starts operation. That is, the completion of the old team's work depends on the start of the new system.
    • Diagram: [Activity A] --SF--> [Activity B]

Step 3: Understanding Dependency Attributes – Lag and Lead

To model real-world situations more precisely, PDM allows adding time offsets to dependencies.

  • Lag: A waiting or delay period added to a dependency.
    • Example (FS with Lag): After Activity A "Pouring concrete" finishes, a 2-day (lag) curing period is required before Activity B "Removing formwork" can start. Represented as FS + 2d.
  • Lead: An overlap period added to a dependency, which can be thought of as "negative lag".
    • Example (FS with Lead): Activity B "Reviewing the report" can start 2 days before Activity A "Writing the report" finishes. This is equivalent to an SS relationship but is often clearer expressed with FS: FS - 2d.

Step 4: Comprehensive Application – Building a Simple PDM Network Diagram

Let's build a simple project schedule using FS relationships.

  • Activities:
    • A: Requirements Analysis (Duration: 5 days)
    • B: System Design (Duration: 8 days)
    • C: Coding (Duration: 10 days)
    • D: Testing (Duration: 5 days)
  • Dependencies:
    • B FS depends on A (Design must start after Analysis finishes)
    • C FS depends on B (Coding must start after Design finishes)
    • D FS depends on C (Testing must start after Coding finishes)

Building the Network Diagram:
[A:5 days] --FS--> [B:8 days] --FS--> [C:10 days] --FS--> [D:5 days]

Calculating Critical Path and Total Duration:
This simple sequential path is the critical path. Total Duration = 5 + 8 + 10 + 5 = 28 days.

Summary
The Precedence Diagramming Method (PDM) visualizes logical relationships between activities using nodes and arrows. Deeply understanding and correctly applying the four dependency types—FS, FF, SS, SF—along with Lag and Lead, is key to creating a logically sound and practical project schedule. This is a prerequisite for Critical Path Method (CPM) calculation and subsequent schedule optimization.