Intelligence and educability
Educability, which refers to the ability to learn school subjects under the ordinary conditions of classroom instruction, is quantitatively indexed by school grades and, more reliably, by scores on standardized tests of scholastic achievement. To what extent does educability depend upon intelligence? The question may seem rather circular, since intelligence tests were devised originally to predict scholastic performance, and scholastic criteria are still paramount in establishing the external validity of many standard intelligence tests. The correlation between IQ and measures of achievement is quite high, ranging from about 0·30 to 0·90 in various studies (the magnitude depending upon many conditions) with an average correlation of about 0·80 when corrected for attenuation (errors of measurement). In other words, something over 60 percent of the true variance in individual differences in scholastic achievement is accounted for by individual differences in intelligence. This evidence has been reviewed in detail by Bloom (1964, Ch. 4) and Tyler (1965, Ch. 5). A pupil's relative standing in achievement is quite stable over the school years, increasing in stability \vith each succeeding year, just as is the case for intelligence measures. Furthermore, the correlations between intelligence and achievement increase with years in school. Bloom (1964, p. 95) reports a correlation of 0·68 (or 0·85 when corrected for unreliability) between 9th year grades and college freshman grades. Another study (Bloom, 1964, p. 102) found a correlation between IQ at age 6 and school achievement at age 13 to be 0·60. And a correlation of 0·42 was found between Stanford-Binet IQ
at age 4 and a scholastic achievement battery at age 13. So there can be no doubt that intelligence tests and achievement tests, even when they have no subject matter content in common, have considerable individual differences variance in common.