A Wild Ass Guess (WAG) is an estimate that is based upon experience, similarity and 'windage' and does not have immediately verifiable data that could be used to substantiate the estimate.
Management requests a rough order of magnitude (ROM) cost estimate, and there isn't time to do all of the necessary research, so you offer a WAG (wild ass guess) estimate scaled off of a previous project that you consider to have a similar complexity.
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