The Planning Fallacy

The tendency to underestimate the time is takes to complete a task.

The Denver International Airport opened 16 months late, at a cost overrun of $2 billion.  The Eurofighter Typhoon, a joint defense project of several European countries, was delivered 54 months late at a cost of £19 billion, instead of £7 billion. The Sydney Opera House may be the most legendary construction overrun of all time, originally estimated to be completed in 1963 for $7 million, and finally completed in 1973 for $102 million. Planners tend to focus on the project itself and underestimate time for sickness, vacation, meetings, and other "overhead" tasks. 

In one study, 37 students were asked to estimate the completion times for their senior theses. The average estimate was 33.9 days. Only about 30-percent of the students were able to complete their thesis in the amount of time they predicted, and the average actual completion time was 55.5 days.

On the opening date for the St. Regis in Kauai: "Originally slated to accept reservations as early as March, the St. Regis public relations department released an official announcement Tuesday evening saying that the opening has been delayed until October 1." (The Garden Island; May 1, 2009)