Mixed longitudinal designs are among the most efficient for the study of growth and
developmental processes. In this approach, one studies several (birth) cohorts, each
for a relatively short length of time, and then links the growth curves for the individual
cohorts together to obtain the growth curve for the entire length of time spanned
by the ages of the subjects in all cohorts. Thus, e.g., in the Nijmegen Growth Study,
three cohorts were each studied for 5 years, the intent being to join the three curves
together to form a single curve covering the entire period from 4 to 14 years of age.
In order for this approach to be valid, there either should be no cohort effects (secular
trends) or the fitted curve must be adjusted in some way to correct for such effects
if they exist. The question thus arises as to how one should test for the presence
of cohort effects and what one should do about them if found. The problems which may
arise using height and weight data from the Nijmegen Growth Study are illustrated.
In particular for girls, height and weight both show cohort effects (at 9.25 years
of age) when the raw data are used. If, however, the observed data are used to estimate
the values at the target age, and these values are used in the comparison, differences
between the cohorts are no longer significant. The problems are further illustrated
using data from a mixed longitudinal data set of cleft lip and palate patients and
data from the National Dutch Growth Study 1980.
Key words
ohort effects - mixed longitudinal design - growth studies
