How Many People Are Mentally Retarded?
We are going to assume that mental function in a general population has a normal distribution or bell-shaped curve, like Figure-A. Levels of mental function are shown in Standard Scores along the horizontal axis and the height of the curve indicates the relative frequency. The crest of the curve represents the most frequent or typical level of mental function, at Std.Score = 0. The curve is exceedingly low at Std.Score 4+, and also exceedingly low at Std.Score -4, because only a very small percent of a general population has either such brilliant or such poor mental function. In any curve like Figure-A, the Std.Scores of 50 % of the population lie to the left of the crest (“below average”), scores of 15.87 % lie at or below the Std.Score -1, and scores of 2.28 % lie at or below Std.Score -2. Either school scores or IQ scores can be converted to Standard Scores (as shown in CNR 1993, Chapter 6). It is self-evident that mental function is a continuum of levels, and that any score dividin
