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Agile Estimating and Planning by Mike Cohn

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Chapter 5. Estimating in Ideal Days

“To achieve great things, two things are needed: a plan, and not quite enough time.”

—Leonard Bernstein

How long is a game of American football?

You could answer that it has four fifteen-minute quarters and is therefore sixty minutes long. Or you could answer that it is somewhere around three hours long. Either answer would be correct in its own way. The difference highlights the distinction between ideal time and elapsed time. Ideal time is the amount of time that something takes when stripped of all peripheral activities. In that sense, a football game takes sixty minutes. Elapsed time, on the other hand, is the amount of time that passes on a clock (or perhaps a calendar). A football game requires around ...

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