The most reliable cost estimate is the definitive estimate. The definitive estimate, sometimes called the bottom-up approach to cost estimating, requires the WBS. As the WBS depicts the complete project scope and all of the work packages to create the project scope, the definitive estimate can examine the cost of each work package to predict the project costs.
Other cost estimating techniques, however, do not have the benefit of the fully-decomposed project scope. The ROM estimate is based on high-level objectives of the project and the budget estimate is based on just the project scope. Only the definitive estimate utilizes the complete WBS for cost estimating. The challenge with the definitive estimate is that it may takes weeks to complete the WBS so it may be weeks before the complete definitive estimate can actually be created.
It's like some lyrics from a Lyle Lovett song: "Would you like a kiss? She said, 'Thank you, no. I'll take some M-O-N-E-Y.'"
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