Educability-and-Group-Differences-1973-by-Arthur-Robert-Jensen

Intelligence and Educability 99
migration of families of abler students out of the rural South,
causing an increasing cumulation of poor students in the higher
grades.
Selective migration, student turnover related to adult employment
trends, and other factors contributing to changes in the
characteristics of the school population, may produce a spurious
PAG when this is measured by comparisons between grade levels
at a single cross-section in time. The Coleman Report’s grade
comparisons are cross-sectional. But where there is no reason to
suspect systematic regional population changes, cross-sectional
data should yield approximately the same picture as longitudinal
data, which are obtained by repeated testing of the same children
at different grades. Longitudinal data provide the least questionable
basis for measuring the PAG. Cross-sectional achievement
data can be made less questionable if there are also socioeconomic
ratings on the groups being compared. The lack of any grade-tograde
decrement on the socioeconomic index adds weight to the
conclusion that the PAG is not an artifact of the population’s
characteristics differing across grade levels.
Another way of looking at the PAG is in terms of the percentage
of variance in individual achievement scores accounted for by the
mean achievement level of schools or districts. If there is an
achievement decrement for, say, a minority group across grade
levels, and if the decrement is a result of school influences, then
we should expect an increasing correlation between individual
students’ achievement scores and the school averages. In the data
of the Coleman Report, this correlation (expressed as the percentage
of variance in individual scores accounted for by the school
average) for ‘verbal achievement’ does not change appreciably
from the beginning of the first school year up to the twelfth
grade. The school average for verbal achievement is as highly
correlated with individual verbal achievement at the beginning
of grade 1 as at grade 12. If the schools themselves contributed
to the deficit, one should expect an increasing percentage of
the total individual variance to be accounted for by the school
average with increasing grade level. But no evidence was found
that this state of affairs exists. The percent of total variance in
individual verbal achievement accounted for by the mean score
of the school, at grades 12 and 1, is as follows (Coleman et al.,
1966, p. 296):