LETTER TO THE EDITOR
Ref: In Defense of PERT-PLUS
In his otherwise excellent hard-hitting January PMWJ article[1] Dr. Paul Giammalvo advocates readers to STOP USING PERT: the standard “PERT” formula te = ( Opt + 4ML + Pess ) / 6 to estimate “Most Likely” and subsequently-calculated “Earliest Expected” times for planning and scheduling activity & project duration; as PERT does not meet the 5 attributes of the Scientific Method, since there are no results that show its successful application.
While it may be ‘nothing more than an unsubstantiated marketing claim’, I maintain the fundamental flaw with PERT is not the formula, but rather its misapplication by those using it; as — by definition — the resultant te is merely a weighted average. Hence, from the outset, the probability for completing individual activities, as well as the overall project, when scheduled by the ‘earliest expected time’ is only 50% — no better than tossing a coin! [Indeed, using the PERT formula is even worse than playing Russian Roulette (RR) where the probability of success (surviving) is 83%, and the probability of being killed “only” 17% (i.e. 1 chance in 6).]
While as yet unproven in practice, my prescription to preclude the inevitable outcome from PERT misapplication is for planners to add two estimated standard deviations to their PERT estimates for a more “Realistic” time (tr); thereby increasing the probability of a successful outcome to 95%.
I developed an Excel template – Figure 1 on the following page — to ‘crunch the numbers’, and also facilitate the ‘negotiating’ process for planners by demonstrating the unlikelihood of attaining any other ‘top down-desired’ or imposed deadlines for project activities. However, not being ‘all-powerful’, if “Top Management” (or the Client) still insist on their deadline or the PERT estimated te, I suggest you ask them to acknowledge the data and accept responsibility for whatever related probabilities they choose to use.
More…
To read entire Letter to the Editor, click here
How to cite this work: Smith, K.F. (2026). In Defense of PERT-PLUS. PM World Journal, Vol. XV, Issue II, February. Available online at https://pmworldjournal.com/wp-content/uploads/2026/02/pmwj161-Feb2026-Smith-in-defense-of-PERT-Plus.pdf
[1] Giammalvo, P. D. (2026). What Do We Do AFTER Calculating Our Average Costs or Durations? PM World Journal, Vol. XV, Issue I, January
