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A New View of the Precedence Diagramming Model

SECOND EDITION

Towards a Vectorized Event Scheduling Technique

By Crispin ‘Kik’ Piney

France


This note does not propose any change to the precedence diagramming method itself. It does however propose a major conceptual change to the way in which the model is viewed.

Background

The standard definition of the PDM is “A technique used for constructing a schedule model in which activities are represented by nodes and are graphically linked by one or more logical relationships to show the sequence in which the activities are to be performed” (Project Management Institute – Practice Standard for Scheduling – Glossary). This is absolutely fine when the logical relationship is “finish-to-start” – i.e. the successor cannot start until a specified time after the predecessor has finished as shown below.

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The definition becomes much less applicable, however, for other types of dependency: take for example “start to finish” where the “successor” cannot finish until the “predecessor” has started (see below). In this case, it is confusing, to say the least, to state that the logical relationship shows the sequence in which the activities are to be performed since it linguistically reverses the arrow of time. In fact, it represents a constraint rather than a sequence for the activities.

pmwj54-jan2017-piney-figure-2

 

 

 


This was the initial insight that led, through a few steps, to the development of a more consistent and simpler approach to schedule modelling. This paper follows the same set of steps towards the final conclusion.

The next section explains that the problem comes from confusing “activities” with “events”.

New View

So what is the answer? The answer is that it is not the activity itself that is the predecessor or successor; it is one of the two possible activity events – i.e. Activity Start of Activity Finish. The four possible relationships FS, SS, SF, FF correspond to all of the combinations of these two events.

This view is also totally compatible with the major deliverable of the precedence diagramming method: the calculation of the possible dates at which these events can occur – i.e. the Early and Late values of the Start and Finish dates.

The only other conceptual step is to realize that an activity can also be represented by an arrow, from Activity Start to Activity Finish. The duration value of this arrow is the duration of the activity.

This new approach makes explaining and understanding the critical path algorithm much more straightforward.

In addition, it integrates the concepts of “activity on arrow” and “activity on node” scheduling representations in a natural manner into a consistent and generalized model.

This is explained in more detail in the following sections:

  • First, the critical path algorithm will be revisited and explained by focusing simply on the “arrows”.
  • This then leads to an analysis in more detail of the role performed by the “arrows”.

More…

To read entire paper, click here

 

Editor’s note: Second Editions are previously published papers that have continued relevance in today’s project management world, or which were originally published in conference proceedings or in a language other than English. Original publication acknowledged; authors retain copyright. This paper was originally published in the March 2012 edition of PM World Today. It is republished here with the author’s permission.

 


 

About the Author

pmwj54-jan2017-piney-photo
Crispin Piney

South of France

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After many years managing international IT projects within large corporations, Crispin (“Kik”) Piney, B.Sc., PgMP is now a freelance project management consultant based in the South of France. At present, his main areas of focus are risk management, integrated Portfolio, Program and Project management, scope management and organizational maturity, as well as time and cost control. He has developed advanced training courses on these topics, which he delivers in English and in French to international audiences from various industries. In the consultancy area, he has developed and delivered a practical project management maturity analysis and action-planning consultancy package.

Kik has carried out work for PMI on the first Edition of the Organizational Project Management Maturity Model (OPM3™) as well as participating actively in fourth edition of the Guide to the Project Management Body of Knowledge and was also vice-chairman of the Translation Verification Committee for the Third Edition. He was a significant contributor to the second edition of both PMI’s Standard for Program Management as well as the Standard for Portfolio Management. In 2008, he was the first person in France to receive PMI’s PgMP® credential; he was also the first recipient in France of the PfMP® credential. He is co-author of PMI’s Practice Standard for Risk Management. He collaborates with David Hillson (the “Risk Doctor”) by translating his monthly risk briefings into French. He has presented at a number of recent PMI conferences and published formal papers.

Kik Piney can be contacted at [email protected]

To view other works by Kik Piney, visit his author showcase in the PM World Library at http://pmworldlibrary.net/authors/crispin-kik-piney/