Estimating, or Rather Underestimating
As shown in Figure 14, Glenn Butts offered the following reasons for under estimating back in 2010.
Figure 14: Glenn Butts Recommendations
None of these suggestions are what you would call "Rocket Science", most of them are little more than "applied common sense". If that is true, why are PMI, IPMA, AACE, APM/APMG et al so unwilling to incorporate these into their Bodies of Knowledge? Wasn't most of this indicated in the original 1987 PMBOK Guide? And here we are 34 years later and we still haven't figured it out. Maybe we are not testing candidates for the right things?
Here's another example. Bent Flyvbjerg's Recommendations from 2007 are shown in Figure 15. Again, there is nothing here that is profound or "new" that we haven't seen before, not just in the past 60 or 70 years but in past hundreds if not thousands of years.
Figure 15: Bent Flyvbjerg's Recommendations from 2007 [click to enlarge]