write:Amos and I coined theterm planning fallacy to describe plans and forecasts that are unrealistically close to best-case scenarioscould be improved by consulting the statistics of si

a distinguished lawyer
a question about areference class: “What is the probability of the defendant winning in caseslike this one?” His sharp answer that “every case is unique” wasaccompanied by a look that made it clear he found my questioninappropriate and superficial. A proud emphasis on the uniqueness ofcases is also common in medicine
in spite of recent advances inevidence-based medicine that point the other way. Medical statistics andbaseline predictions come up with increasing frequency in conversationsbetween patients and physicians. However
the remaining ambivalenceabout the outside view in the medical profession is expressed in concernsabout the impersonality of procedures that are guided by statistics andchecklists.The Planning FallacyIn light of both the outside-view forecast and the eventual outcome
theoriginal estimates we made that Friday afternoon appear almostdelusional. This should not come as a surprise: overly optimistic forecastsof the outcome of projects are found everywhere. Amos and I coined theterm planning fallacy to describe plans and forecasts that are unrealistically close to best-case scenarioscould be improved by consulting the statistics of similar casesExamples of the planning fallacy abound in the experiences ofindividuals
governments
and businesses. The list of horror stories isendless. In July 1997
the proposed new Scottish Parliament building inEdinburgh was estimated to cost up to £40 million. By June 1999
the budget for the building was £109 million. In April 2000
legislatorsimposed a £195 million “cap on costs.” By November 2001
theydemanded an estimate of “final cost
” which was set at £241 million.That estimated final cost rose twice in 2002
ending the year at