Project Management – Precedence Diagramming Method & Leads and Lags

Precedence Diagramming Method (PDM)

PDM is a technique used to show a logical relationship between activities. It shows the sequence in which those activities are needed to be performed. Activities are categorized into predecessor and successor, and they have four types of dependencies between them.  By definition, a predecessor activity comes before a successor activity, and a successor activity comes after a predecessor activity.

The four types of dependencies are:

  • Finish to Start (FS):
    • A successor cannot start until the predecessor activity has finished
  • Finish to Finish (FF):
    • A successor activity cannot finish until the predecessor activity has finished
  • Start to Start (SS):
    • A successor activity cannot start until the predecessor activity has started
  • Start to Finish (SF):
    • A successor activity cannot finish until the predecessor activity has started

table

pdm relationship diagramsLeads and Lags

Leads and lags are used by project management teams to accurately depict the timing of activities with respect to each other. It shows the dependencies between tasks and helps in determining any schedule conflicts or delays.

Lead is the amount of time a successor activity can be advanced with respect to a predecessor activity. Below is an example of a Finish to Start relationship where the successor activity can start 2 weeks before predecessor activity finishes.leaddiagram

Lag is the amount of time a successor activity will be delayed with respect to a predecessor activity. Below is an example of a Finish to Start relationship where the successor activity starts 3 weeks after predecessor activity finishes.
lagdiagram

 

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