The PMBOK 5thEdition is open for review and the people who have written the Time and Cost Chapters are so ill-informed they don’t even appreciate the difference between a Modified BETA distribution curve, and the PERT Scheduling methodology that used this inaccurate but mathematically simple distribution to simplify the probability calculations inherent in the PERT methodology.
The standard is open for public comment – you don’t need to be a PMI member to have your say; but I do think the professional planning and scheduling community need to put in a major effort to get this ridiculous error removed from the PMBOK. You can participate in the public commentary via: http://www.pmi.org/PMBOK-Guide-and-Standards.aspx [1]
For those who don’t know our history, PERT was a variation of the ‘Activity-on-Arrow’ scheduling format developed by the POLARIS SPO in 1957, that used 3 point estimates and a Modified BETA Distribution to determine the probability of achieving project milestones and the overall completion date. The Modified BETA formula used in PERT is (A + 4B + C)/6.
The CPM methodology developed around the same time by Du Pont Numiers also used the ‘Activity-on-Arrow’ scheduling format but with a single deterministic task duration. The now standard Precedence style of networking was described in a paper published in 1961. For more on the history of scheduling see ‘A Brief History of Scheduling’: http://www.pmi.org/PMBOK-Guide-and-Standards.aspx [1]