Once work packages and activities are known, the team estimates how long each will take and what it will cost. Estimates are best made by the people who will do the work, and they should state their assumptions and a range, because every estimate is uncertain. Accuracy usually improves as the project progresses, which is why early estimates are rough (a rough order of magnitude, or ROM) and later ones are refined into budget and definitive estimates. This is called progressive elaboration. Present an estimate as a range, such as 8 to 10 weeks, rather than a single date whenever you can, so stakeholders understand the uncertainty instead of treating a guess as a promise.
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