With all due respect your analogy with weight and height is a bad analogy. A much better analogy would be blood sugar level. For many healthy people its sorta

With all due respect your analogy with weight and height is a bad analogy. A much better analogy would be blood sugar level. For many healthy people its sorta constant-ish. The fact that its not nearly constant for a subset of people doesn't prove the test is therefore worthless. It just proves some people have untreated diabetes. The test is not worthless because it can be used in diabetes screenings, unless the point of the test was to tap people like maple syrup trees or something really weird like that. A major problem with the test is we know people are not terribly good at self introspection, we're all above average drivers and lovers and thinkers, all of us. I am good enough at self introspection to tell I'm pretty well introverted, yet I can imagine someone not so observant or self aware or perhaps neurotic about the whole topic, getting a fairly random score. Or I'm bad enough at self introspection to not realize I'm no good at it at all, and I'm actually a closet extrovert, although I find that rather unlikely (although I would, wouldn't I, if I failed at introspection). In that way as a secondary measurement I'd propose theoretically that the wider the variation in test results, the worse the person would score in some kind of self introspection ability test. It seems a reasonable hypothesis. You'd need correction factors to deal with people having diagnosed or undiagnosed swings in brain neurotransmitters or use of addictive substances (including sugar and caffeine). But it could be run as a study.