If we're going to talk about the formality of methods, let's remember that "collection of positive anecdotes with back-of-envelope guesstimates" is not what's m

If we're going to talk about the formality of methods, let's remember that "collection of positive anecdotes with back-of-envelope guesstimates" is not what's meant by a "business case". An ROI or IRR calculation might not have anything like the elegance of a proof, but it's a method and it's formalised.