The precedence diagramming method (PDM) or Activity on Node (AON) is the most commonly used method for arranging the project work visually.
Within the PDM technique, tasks are depicted as nodes, outlining their logical relationships or dependencies. This visual representation elucidates the sequential logic guiding the order in which activities are intended to be executed.
Precedence Relationship
PDM includes four types of dependencies or logical relationships called Precedence Relationship
| Finish-to-Start (FS): This relationship signifies that the successor activity cannot start until the predecessor activity is finished. It represents a typical sequential order. | Finish-to-Start (FS) Example: Activity A: Setting up project milestones Activity B: Developing project timeline Relationship: Activity B (Developing project timeline) cannot start until Activity A (Setting up project milestones) is finished. |
| Start-to-Start (SS): In this relationship, the successor activity cannot start until the predecessor activity has commenced. Both activities happen simultaneously but have different start times. | Start-to-Start (SS) Example: Activity A: Researching market trends Activity B: Designing product features Relationship: Activity B (Designing product features) can start as soon as Activity A (Researching market trends) begins. |
| Finish-to-Finish (FF): This relationship indicates that the successor activity cannot finish until the predecessor activity is finished. Both activities conclude at the same time, but they may have different start times. | Finish-to-Finish (FF) Example: Activity A: Writing software code Activity B: Conducting software testing Relationship: Activity B (Conducting software testing) cannot finish until Activity A (Writing software code) is finished. |
| Finish-to-Finish (FF): This relationship indicates that the successor activity cannot be finished until the predecessor activity is finished. Both activities conclude at the same time, but they may have different start times. | Start-to-Finish (SF) Example: Activity A: Ordering raw materials Activity B: Assembling product Relationship: Activity B (Assembling product) cannot finish until Activity A (Ordering raw materials) has started. |
Activity Types
Predecessor Activity – It is the activity that logically comes before a dependent activity in a schedule. In the below figure, Activity A = Predecessor Activity
Successor Activity – It is a dependent activity that logically comes after another activity in a schedule. In the below figure, Activity B = Successor Activity
Another Important area while creating a PDM is Dependencies
Mandatory dependencies are those that are legally or contractually required or inherent in the nature of the work and involve physical limitations: you can’t begin building your house until your foundation is in place.
Discretionary or subjective dependency is those that are decided by the project manager to adapt to the reality of their environment or for the sake of efficiency also known as Preferred logic; preferential logic, or soft logic.
Example of Dependencies
- Mandatory/Must have: legal, regulations, contracted
- Discretionary: Good to have: Best Practices, Standards or resource availability
- External: Don’t have any control over it: Delivery, Procurement, permits
- Internal: can be managed as this is within a project
Further Readings
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