That’s not to say that guessing is fine. Not in the long run.
At the beginning, when you can’t know, it’s good to put guesses into the thinking – what else do you have? – but it’s important to identify them guesses, to remind everyone they’re guesses … and to replace them with actual knowledge as soon as you can.
The alternative, that you kinda-sorta forget that you plucked them out of your head (or worse, that you keep it a secret and bluff your way through questioning), sinks you in quicksand. Guesses can take on a sticky substance, people referring to them, building them into plans and models that solidify around your ankles.
So take a guess, use it as a place holder and mark it with highlighter, for now. Then clang it to the bin, loudly, as soon as you possible can.
Skippy strategy: Highlight, then wipe away your guesses.
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