12 October 2005

PDMs - Precedence Diagram Method


The Precedence Diagramming Method (PDM) was developed in the early 1960s by H.B. Zachry in cooperation with IBM.

It has largely replaced Arrow on Node diagramming. PDMs represent activities as boxes that are assigned properties of the activities they represent.

See here for a good Article on benefits of PDMs. PDMs are popular and useful because they can:
  • Help find the critical path
  • Help define the amount of time required
  • You can use them to crash projects
  • You can use them to flatten resources
  • Define dependencies/precedence
  • Identify lead and lag times
The fundamentals of a PDM element are represented in the below diagram:
  • Precedents or dependencies
  • Earliest start
  • Estimated duration
  • Earliest finish
  • A description of the activity
  • Latest start
  • Float time
  • Latest finish




In practical terms revisiting the basics has enabled me to be more effective in plotting schedules into Project. I used to tackle tasks/activities in a sloppy fashion, but since walking through the basics I have had to put together two project schedules, and I took a more disciplined approach to each and have a much more satisfactory result, especially when the changes start rolling in.
An article called "Four scheduling exerts" by Richard Korman with Stephen H. Daniels at the PMI website discusses how the PDM is often biased and what should be done about it. The essence of their answer: make sure people are properly trained and experienced. And the American Society for the Advancement of Project Management also has an article "Let's scrap the PDM" which suggests node and arrow plans are better.