

Why are software development task estimations regularly off by a factor of 2-3? - mlinsey
http://www.quora.com/Engineering-Management/Why-are-software-development-task-estimations-regularly-off-by-a-factor-of-2-3?q=why+are+software+project

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brg
Because software development is an act of invention. While experience and
education often tell us that something is possible to accomplish, the route to
the end is unknown.

The metaphor Michael Wolfe provides is good. But software scheduling isn't so
much about diversions and stumbles as it os about the trials of invention and
artisanship.

