Excellent
Unlike other iPhone (or really, most popular "do-it-yourself") IQ tests which focus solely on one area or another, this one gives questions in all areas intelligence must be measured in.
Obviously, this cannot take the place of a legitimate psychiatrist, but it does give a good ballpark image of the users IQ. For example, taking into account test deviance, my own was reported by psychiatrists as between 148 and 153, and this test reported 144 (though I understand it does not test past 145). This is less than one-half a standard deviations difference (the standard deviation of IQs is 15), which makes this test, in my experiences, a relatively decent measure.
A note of warning, though - knowing your own IQ changes nothing in your life. Youre just as smart as you were before you knew.
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Dr. Reichel's IQ Test