Part 86: Precedence Diagramming Method
(Project Schedule Management: Sequence Activities)

  • The Precedence Diagramming Method is used to construct a schedule model
  • Each activity in the model is represented by a node
  • Each activity is connected to other activities logically
  • The Model shows us the sequence for performing the activities
  • There are four possible relationship types in the PDM
    • Finish-To-Start (FS)
      • This means that the second activity can’t start until the first activity is complete.
      • For example, we can’t start framing the house until the concrete is dry.
    • Finish-To-Finish (FF)
      • This means that the second activity can start (but can’t finish) until the first activity has finished.
      • For example, we can start painting the house before the drywall is complete (although this is not typical).  However, we can’t finish painting until all the drywall is installed.
    • Start-To-Start (SS)
      • This means that the second activity can’t start until the first activity has started (but the first activity doesn’t have to finish for the second activity to finish).
      • For example, we can’t install the furnace until the plumber/gasfitter shows up to connect the gas line.  Once the gas line is installed, we can finish installing the furnace, even if the plumber isn’t finished the plumbing.
    • Start-To-Finish (SF)
      • This means that the second activity can’t finish until the first activity has started (but the first activity doesn’t have to finish for the second activity to finish).  Note that the second activity can start before the first activity.
      • For example, the electrician can start installing light fixtures before the drywall and paint is complete.  But the electrician can’t finish installing switches and covers until the drywall and paint is complete, because the holes must be covered up.
  • In the PMP, most activities are Finish-To-Start, and sequential.  Each activity has one predecessor and one successor.
  • In the real world, relationships can be complicated. 
    • Remember that an activity can connect to several or dozens of other activities, not just one or two.
    • An activity can have different relationships with different activities.
    • When activities are complicated, an activity can have two or more relationships with the same activity.  We want to select the relationship with the largest impact.