Precedence Diagram
A Diagram in which Activities are represented as Nodes, linked by Arrows which represent sequence in which activities stop or start is called precedence diagram.
PDM includes four Dependencies (Precedencies) or Logical Relationships between Activities
- Finish-to-Start
- Start-to-Start
- Finish-to-Finish
- Start-to-Finish
Predecessor Activity is the one that logically (chronologically) comes before the Successor Activity
A Dependent Activity is the one that is dependent on another Activity for Start or Stop. Usually, but not always, the Successor Activity
Finish-to-Start (FS)
- Dig Foundation
- Pour Concrete
Pouring of Concrete (successor) can only Start after Foundations (predecessor)have been dug
Start-to-Start (SS)
- Pour Asphalt
- Level & Compress the Road
- Levelling & Compressing of the Road (successor) can only Start after the Pouring of Asphalt (predecessor)has Started
Finish-to-Finish (FF)
- Write a Document
- Edit the Document
Editing the Document (successor)can only Finish if Writing the Document (predecessor)has finished
Start-to-Finish (SF)
- Old Account Payable
- New Account Payable
Old Account Payable (predecessor)can be Shut Down only once the New Account Payable (successor)Starts
Mandatory & Discretionary Dependencies
Mandatory Dependencies
Physically or Legally binding Dependencies. Like, Superstructure can only be erected AFTER Foundations have been built
- Also known as Hard Logic
Discretionary Dependencies
Dependencies desired for any particular reason, like Good Practice. Plumbing and Electrical work can be done in parallel but Good Practice is to Start Electrical work after Plumbing work has completed, to reduce Risk
- Also known as Preferred or Soft Logic
Internal Dependencies
Within the control of Project Team. Like, a machine must be Assembled before it can be Tested.
External Dependencies
Dependencies not within the control of the Project Team, one of the Activities being outside the Project Domain. Like, a customer wants some special Spanish tiles in bathrooms for which an order has been placed on the OEM. The tiles must be Shipped before they can be Installed.
Lead vs Lag activity
Lead
Amount of Time a Successor Activity can be Advanced with respect to the Predecessor Activity, for any reason. E.g. Start Editing the Document 2 weeks before Scheduled Completion Date of its Draft
Lag
Amount of Time a Successor Activity is Delayed with respect to the Predecessor Activity, for any reason. E.g. start 1stFloor Masonry Work 10 days AFTER Pouring Ground Floor Slab, to allow for adequate curing of Slab
