Just a thought, from RESULTS ORIENTED
How overconfident are your project estimates?
Numerous cognitive research studies have found that people are consistently overconfident in their judgment by about 20%.
And if you're a manager, your margin of "optimism" is probably 40% or more.
Business school professors J. Edward Ruse and Paul J. H. Schoemaker studied 1,000 American and European managers and found the group to be correct only 30% to 60% of the time.

Good News: Knowing that your estimates will be inaccurate is the first step. Using basic measurements, reviewing those measurements, as well as estimating experience, can improve your accuracy.

Source: An article from Baseline Magazine