Tag Archives: PERT

Scheduling Acronyms – use the correct terms!

Critical path scheduling has only been around for 60 years, is well documented by the originators of the discipline and central to the practice of project management. However, through ignorance, overt commercialism or laziness, far too many scheduling professionals continue to confuse the terms and degrade the practice.

If we can’t use the same correct term consistently for a function or process in scheduling why should anyone else take us seriously. The originators of the various concepts knew what they called each of the items discussed below, it is both professional and polite to respect their intentions and legacy.

CPM = Critical Path Method. (Also called CPA – Critical Path Analysis) This term emerged in the 1960s to describe the two variants of CPM, ADM and PDM. CPM uses a single deterministic duration estimate for each activity (or task) to calculate the schedule duration, activity start and finish dates, various floats and the ‘critical path’. CPM focuses on the activities.

ADM = Arrow Diagramming Method. Also called AOA (Activity on Arrow).  ADM was the first of the CPM techniques developed by Kelley and Walker in 1957.  This style of network diagramming has largely faded from use.

Activity-on-Arrow Diagram

Activity-on-Arrow Diagram


PDM = Precedence Diagramming Method. Also called AON (Activity on Node).  PDM was the second of the CPM techniques developed by Dr. John Fondahl, and published in 1961.  PDM is the standard form of CPM networking used today.


Precedence diagram


PERT = Programme Evaluation Review Technique. PERT was developed by the US Navy in 1957 in parallel with CPM and used an identical ADM network.   PERT differentiates from CPM in several ways.  Its focus in on the probability of achieving an event (eg, the completion of a phase or activity), the expected duration of each activity is calculated from three time estimates using a ‘modified Beta distribution’ (optimistic, most likely and pessimistic).  The ‘PERT Critical Path’ is calculated using the ‘expected’ durations and very simplistic probability assessments can be made based on the variability in the three estimates (but only on a single path).  PERT calculations for the ‘expected’ durations can be applied to PDM networks but are only of value if all of the links are ‘Finish-to-Start’. PERT is simplistic and significantly less accurate than the modern Monte Carlo analysis. For more on this see: Understanding PERT.


Calling any deterministic CPM schedule a PERT Chart is simply wrong; PERT is defined by three time estimates! Using PERT when you could use Monte Carlo is stupid – the information generated is less accurate. And inventing new names for existing processes is confusing and damaging.

Valuing Project Procedures

I am frequently asked to quantify the value of improving an organisations project management capabilities or how to establish the ROI for a new PMO.

Whilst these questions are sensible they are nearly impossible to answer. Certainly there are strong indicators of the value generated by an effective PMO, this has been demonstrated repeatedly in studies by KPMG, PWC and others (Download the PMO studies).

OPM3 is more difficult. The most useful option is a comparison with CMMI.  The larger user base for CMMI makes statistical analysis possible and demonstrates a consistent value proposition for improving organisational maturity and capability (see more on OPM3).

The question is can the generic data generated by these studies be translated to a specific proposal in a single organisation. Unfortunately the answer is no.  On average an organisation can expect a significant return on monies invested in PMOs and improving project, program and portfolio management maturity but as risk practitioners know only to well, on average, nothing is average.  Some situations will fail, other will generate stellar returns.

This is not a new problem.  In June of 1962 the USA Dept. of Defense promulgated PERT/COST as a new general purpose management system for use on major military system acquisition programs. In 1964 a major study was undertaken by The Mitre Corporation to investigate the question of how to evaluate the design of the PERT/COST management system. This study still makes interesting reading today.

The overarching conclusions in the report were:

  • That there is no single, simple straightforward way of deriving value judgments as to the PERT/COST system design, or probably any other general purpose management system.
  • The interrelationships between a management system and the quality of its implementation operation (including the capability of the managers who use it), presents serious difficulties in the assessment of the value of the management system alone.
  • The value of the system is intimately related to both the quality of its implementation and the capability and willingness of the appropriate managers to use it.
  • An evolutionary approach is a good way to evolve the development of the system capability in an orderly fashion over period of time. It is ideal in cases where the ultimate capability to be required of the system cannot be precisely defined, but where the direction toward which increasing system capabilities should be oriented are predictable.

My post on Cobb’s Paradox asked the question why do executive managers allow poor quality systems to exist in their organisations. Possibly one answer is the difficulty of generating a simple investment proposition discussed in this post.

Better informed executives are capable of bypassing set minimum ROI values or payback periods, focusing instead on the demonstrated competitive advantage to be gained by selecting the right projects and programs to do, then doing them right!  The challenge for project management professionals in other organisations is making the necessary information available in ways that can be received and understood by the executives.

In conclusion, Harry S Truman said The only new thing in the world is the history you don’t know.”  To help you avoid this problem, the 1964 Mitre Report, authored by R. L. Hamilton, can be downloaded from the link (Handle) on  http://oai.dtic.milAD0603425

PM History papers updated

Working on my paper for PMOZ 2010, Seeing the Road Ahead – the challenge of communicating schedule data  has required me to re-visit two key papers and augment them with new information and materials discovered in the last few years.

A Brief History of Scheduling – Back to the Future has had quite a lot of new materials incorporated. I am now confident this paper accurately lays out the development of scheduling and in particular, the origins of PERT and CPM.

The Origins of Modern Project Management  has had a few new footnotes included an links the development of modern project management to its roots the the spread of scheduling in the early 1960s.

Both updated papers are available for downloading and I have most of the reference materials available for anyone interested in further research into these topics.

The PMOZ paper will be available after publication in a couple of weeks. For more on the PMOZ conference see: http://www.pmoz.com.au/