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

294 Educability and Group Differences
are then expressed as deviates of the normal distribution. In
essence, standardization is the process of re-scaling raw scores in
terms of the mean and standard deviation of the so-called normative
population. The normative population may or may not
include one or another racial subpopulation. And some normative
samples are more representative of one geographical region or its
racial composition than another. It is possible to obtain separate
norms for whites and Negroes, and it is possible to have norms
based on the combined groups each represented proportionally
to their frequency in the general population. Various standardized
tests have used one or another of these methods. The important
point, however, is that it makes absolutely no real difference in
terms of the rank order of individuals or of subpopulations on the
test, and it has no effect whatsoever on the predictive validity of
the test, any more than it lowers a patient’s temperature to change
from a Fahrenheit to a Centigrade thermometer. We are simply
assigning different numbers to the same relative differences. For
example, a test standardized exclusively in the white population
and given a mean of 100 and an SD of 15 will show a mean of
approximately 85 and an SD of 13 when given to a representative
sample of the Negro population. If we standardize the test in a
combined sample of whites and Negroes, represented in the
proportions of their frequencies in the general population of the
United States (approximately 89 percent whites, 11 percent
Negroes), the general mean IQ will still be set at 100 and the SD
at 15. But on this scale the Negro mean will be 87-1 and the white
mean will be 101-6, for a difference of 14*5 IQ points. But the
percentage of Negroes exceeding the white median (i.e., median
overlap) will remain exactly the same, viz., 14-2 percent. (Median
overlap has this advantage as a measure of group differences - it
is invariant regardless of the scale of measurement.) Thus, it is
apparent that re-standardizing or re-scaling IQ tests would make
no essential difference in Negro-white comparisons.
Having separate sets of norms for Negro and white populations,
each given a mean of 100 and SD of 15, could only impair the
predictive validity of the tests when they are used in mixed
populations. This is because intelligence tests standardized on
whites or on Negroes and whites together have the same predictive
validity for Negroes as for whites (see Jensen, 1971d and Stanley,
1971, for reviews for this evidence). In other words, the tests