Welcome, Oliver Oddball:
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The IQ score given for Orson Welles is 146, in the top one or two percent tested by present reckoning, which seems reasonable, if we note the praise, attention and stimulation he was exposed to in "home-schooling" as a small child. But no source is listed for that score, and we are not told what test was used. The Stanford-Binet Test in its combined, standardized form dates only from 1916, and would have been judged "new-fangled" in the early 1920's.
We do know that, the year after his mother's death, 1925, Welles was sent by his guardian, Dr. Maurice "Dadda" Bernstein, to Madison, Wisconsin, where he was examined and run through a battery of tests by a well-known psychologist, Dr. Frederick Mueller, preparatory to selecting a school for the ten year-old. Interestingly, the recommendation of Dr. Mueller was that he be sent to a good, normal public school, Washington Grade School, in Madison. That choice did not work out, and Welles' father, Dick Welles, subsequently picked Todd School, where the family's older "backward" son, Richard Welles, had attended for awhile, unsuccessfully, before being institutionalized.
Orson Welles flowered at Todd School, and became perhaps its most famous success.
And so, as almost always with Welles, there is a little ambiguity in the record.
Hope that helps.