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"Much empirical analysis in The Bell Curve is based upon two data sets, the National Longitudinal Survey of Youth (a large sample of American youth, aged 14-22 in 1979, who have been followed annually since then) and the Children of the NLSY, which matches women in the NLSY with their children. Both data sets contain good measures of cognitive ability, but, say Hauser and Carter, are used poorly by Herrnstein and Murray. Most of the original analysis in the book consists of graphical displays of reduced-form logistic or linear regression equations in which some measure of educational or socioeconomic attainment, contact with the criminal justice system, or child-rearing success has been regressed on two variables, AFQT score in the IQ metric, adjusted for age at administration, and a composite measure of the socioeconomic status (SES) of the family of orientation. This measure is limited in content to father's and mother's educational attainments, father's occupational status, and family income in 1979, the first year of the NLSY. This is a minimally adequate specification, but it tends to understate the effects of social background by omitting such variables as number of siblings, intact family, rural or metropolitan origin, and regional origin. Thus, in Herrnstein and Murray's analysis, the social background variable becomes a straw man, largely used to highlight the effects of ability. From the study of stratification, it is known that the explanatory power of measured social background is modest, but it is also known that the effects are important and worth understanding. No measures of the explanatory power of the equations are reported in The Bell Curve, so that the inexpert reader never learns tha t most of the variation remains unexplained.

The following is an explanatory excerpt from this article:
"Much empirical analysis in The Bell Curve is based upon two data sets, the National Longitudinal Survey of Youth (a large sample of American youth, aged 14-22 in 1979, who have been followed annually since then) and the Children of the NLSY, which matches women in the NLSY with their children. Both data sets contain good measures of cognitive ability, but, say Hauser and Carter, are used poorly by Herrnstein and Murray. Most of the original analysis in the book consists of graphical displays of reduced-form logistic or linear regression equations in which some measure of educational or socioeconomic attainment, contact with the criminal justice system, or child-rearing success has been regressed on two variables, AFQT score in the IQ metric, adjusted for age at administration, and a composite measure of the socioeconomic status (SES) of the family of orientation. This measure is limited in content to father's and mother's educational attainments, father's occupational status, and family income in 1979, the first year of the NLSY. This is a minimally adequate specification, but it tends to understate the effects of social background by omitting such variables as number of siblings, intact family, rural or metropolitan origin, and regional origin. Thus, in Herrnstein and Murray's analysis, the social background variable becomes a straw man, largely used to highlight the effects of ability. From the study of stratification, it is known that the explanatory power of measured social background is modest, but it is also known that the effects are important and worth understanding. No measures of the explanatory power of the equations are reported in The Bell Curve, so that the inexpert reader never learns that most of the variation remains unexplained.