It seems that the rhythm to developing software in XP marches to a beat marking TwoWeeks.  What is magic about TwoWeeks?
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	* Nobody can estimate anything with any accuracy of duration greater than TwoWeeks.
	* How wrong can you be in TwoWeeks?

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I tried OneWeek instead of TwoWeeks in a project recently, and I think OneWeek is just too short for useful stories to emerge. We ended up tacking simpler stories first since they fit the week boundary, instead of the riskier ones.
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See ProjectVelocity