Precedence Diagramming Method
The Precedence Diagramming Method (PDM) is a technique used in project management to construct a schedule network diagram that graphically represents the sequence of project activities. In PDM, activities are depicted as nodes (boxes), and the dependencies between these activities are represented with arrows. This method allows project managers to visualize the flow of activities and understand the relationships and dependencies that exist within a project. PDM supports four types of logical relationships between activities: 1. **Finish-to-Start (FS):** The successor activity cannot start until the predecessor activity has finished. This is the most common relationship used in project schedules. 2. **Start-to-Start (SS):** The successor activity cannot start until the predecessor activity has started. This relationship is useful when activities can occur in parallel to some extent. 3. **Finish-to-Finish (FF):** The successor activity cannot finish until the predecessor activity has finished. This ensures that two activities are completed simultaneously or that one cannot conclude before the other. 4. **Start-to-Finish (SF):** The successor activity cannot finish until the predecessor activity has started. This is the least common relationship and is used in specific situations where the finish of an activity depends on the start of another. By utilizing PDM, project managers can identify the critical path, which is the longest sequence of activities that determines the minimum project duration. Understanding the critical path is essential for effective time management, as delays in critical path activities will directly impact the project's completion date. PDM also facilitates the identification of leads and lags, allowing for more flexible scheduling and optimization of resource allocation. Overall, the Precedence Diagramming Method is a fundamental concept in schedule network analysis that aids in planning, organizing, and controlling project activities to achieve timely project completion.
Precedence Diagramming Method (PDM) Guide
What is Precedence Diagramming Method?
The Precedence Diagramming Method (PDM) is a technique used in project scheduling that graphically displays project activities and their relationships. PDM is the most commonly used method for creating a project schedule network diagram in modern project management software.
Why is PDM Important?
PDM is critical because it:
- Visually represents the logical relationships between activities
- Helps identify the critical path (the sequence of activities that determines the project duration)
- Reveals scheduling constraints and dependencies
- Enables calculation of float/slack time
- Facilitates resource planning and optimization
- Supports schedule compression techniques like fast-tracking and crashing
How PDM Works
In PDM, activities are represented as nodes (boxes), and relationships between activities are shown as arrows connecting these nodes. This is also known as the Activity-on-Node (AON) method.
The Four Types of Dependencies in PDM:
1. Finish-to-Start (FS): The most common relationship. The successor activity cannot start until the predecessor activity finishes. Example: Walls cannot be painted until they are built.
2. Start-to-Start (SS): The successor activity cannot start until the predecessor activity starts. Example: Once concrete pouring begins, smoothing can start shortly after.
3. Finish-to-Finish (FF): The successor activity cannot finish until the predecessor activity finishes. Example: System testing cannot finish until debugging is complete.
4. Start-to-Finish (SF): The successor activity cannot finish until the predecessor activity starts. Example: Night shift security cannot end until day shift security begins.
These relationships may also include leads (acceleration) or lags (delays). For example, FS+5 indicates that the successor activity can start 5 time units after the predecessor finishes.
Steps to Create a PDM Network:
1. Identify all project activities
2. Determine dependencies between activities
3. Establish the type of relationship for each dependency
4. Draw the network diagram with nodes and arrows
5. Add duration estimates to each activity
6. Calculate early start, early finish, late start, and late finish times
7. Identify the critical path and calculate float
Exam Tips: Answering Questions on PDM
1. Understand the terminology: Be clear about nodes, dependencies, critical path, float, and the four relationship types (FS, SS, FF, SF).
2. Practice network calculations: Know how to calculate early and late start/finish times, and total float for activities.
3. Identify the critical path: Remember that activities on the critical path have zero float and determine the project duration.
4. Recognize dependency implications: Understand how each type of dependency affects the schedule.
5. Watch for leads and lags: Pay attention to any specified leads or lags as they impact the schedule calculations.
6. Focus on common exam scenarios: Practice identifying the impact of adding or removing dependencies, changing durations, or adding constraints.
7. Understand float concepts: Know the difference between total float, free float, and project float.
8. Apply schedule compression techniques: Be familiar with how fast-tracking (parallel activities) and crashing (adding resources) affect the network diagram.
9. Read questions carefully: PDM questions may ask you to identify impacts of changes or find the optimal sequence of activities.
10. Draw it out: For complex questions, sketch the network diagram to visualize the scenario rather than trying to solve it mentally.
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