Prevalence of a group defined by IQ of 50 to 69 reflects largely the 'normal' population distribution of IQs. For a test of mean 100, standard deviation 15, 2.27 per cent of the population will fall below IQ 70 plus a small effect (mostly IQ < 50) from specific pathologies. The few populations providing data confirm this with figures of 25 to 30 per 1000. Intelligence tests should be validated for specific populations. Test means and standard deviations should be known from recent studies, because population means, to which all IQ scores are related, change over time. Data are not available for most populations! Because measured intelligence in populations varies, studies of the effects on intelligence of environmental hazards (e.g. birth trauma or lead) must assess outcome in relation to population norms in a related cohort of births.
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