Basically, what you do is you compare each age group IQ to the standardization sample.
According to Kaufman (2005, p. 172), "Parker (1986) had the clever idea of examining the comparative performance of year-of-birth cohorts by equating the standardization samples of the Wechsler-Bellevue I, WAIS, and WAIS-R."
Kaufman describes his procedure for a longitudinal study as follows (p. 172f.):
The WAIS and WAIS-R standardization samples are quite similar to each
other, each matching relevant Census data on numerous key variables.
They differ in that the data were collected 25 years apart, in
approcimately 1953 and 1978. Thus, several cohorts in the WAIS sample
are also represented in the WAIS-R sample. For example, adults born in
the 1909-1913 cohort were tested at ages 40-44 in 1953 (on the WAIS),
and again at ages 65-69 in 1978 (on the WAIS-R). To the degree that
the two samples are comparable, a comparision of the test performance
of 40- to 44-year-olds on the WAIS with that of the 65- to
69-year-olds 25 years later on the WAIS-R represents a longitudinal
comparison of adults from the same cohort.
Before making the comparisons, he [Kaufman] verified empirically that
the independent samples were extremely well matched and comparable
within each of the four cohorts on the important variables of gender,
race ..., geographic region, and educational attainment. Then, he had
to convert sums of scaled scores on the WAIS and WAIS-R to a common
yardstick to permit age-by-age comparisons, and chose to use the norms
for ages 25-34 for all adults in the study. Next, he had to control
for the fact that different tests (WAIS vs. WAIS-R) were administered
at the two points in time. Conceptually, these two adult scales are
interchangeable ..., but because of the "Flynn effect" the WAIS-R
yields lower IQs. Kaufman added 6 to 6½ points to each WAIS-R IQ (the
median differences from 20 studies totaling over 1.300 subjects)
(Kaufman, 1990, Table 3.13) to convert these IQs to WAIS IQs. These
"corrections" to the WAIS-R IQs helped answer the crucial question,
"How many IQ points higher would adults have scored had they been
administered the WAIS instead of the WAIS-R in 1978?" Finally, he
applied a time-lag correction to control for cultural change during
the 25-year span, just as Owens (1966) did in his Iowa State study.
Adjustment for cultural change requires a comparison of the IQs earned
by adults of the same age in 1978. The 1909-1913 cohort, for
example, was 40 to 44 years old n 1953. This group was compared to
adults aged 40-44 in 1978 to determine how cultural changes have
affected test scores for this age group. Similar time-lag comparisons
were conducted for each of the other three cohorts who, in 1953, were
ages 20-29, 30-39, and 45-49. The analyses showed that cultural change
affected each of the four cohorts about equally, producing about a
3-point IQ gain on the Verbal and Full Scales and about a 5½-point
gain on the Performance Scale, presumable due to some type of
culture-related change between 1953 and 1978 that affected all adults
who were between the ages of 20 and 49 in 1953. Kaufman (1990)
adjusted the estimated WAIS IQs earned by each cohort in 1978 for
these time-lag effects to remove the influence of cultural change.
- Kaufman, A. S. (2005). Assessing adolescent and adult intelligence (3rd ed.). Wiley.
- Parker, K. C. H. (1986). Changes with age, year-of-birth cohort, age by year-of-birth interaction, and standardization of the Wechsler adult intelligence tests. Human Development, 29, 209-222. doi:10.1159/000273048