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
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.
Summary
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.
Pat Weaver
Lynda Bourne
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