Effects of measurement aggregation on predicting externalizing problems from preschool behaviour

In long‐term studies of psychological development, the initial assessment of etiologically significant child behaviours is often carried out at a single point in time only. However, one‐time assessments of behaviour are likely to possess limited reliability, leading to attenuated longitudinal correlation coefficient magnitudes. How much this bias might have affected behavioural continuity estimates in longitudinal research is presently unknown. Using a data set from the Mauritius Child Health Project, we particularize the attenuating effects of single‐occasion behavioural assessments on consistency estimates of impulsive–aggressive behaviour over time. Specifically, two nursery teachers provided 15 consecutive weekly ratings of the aggressive behaviour of 99 four‐year‐old children. The same children were reassessed for the presence of externalizing behaviour problems at the ages of 8 and 10. There were substantial increases in both reliability and predictive correlation coefficient magnitudes when the preschool scores were aggregated across several weekly ratings. A further increase resulted after the two outcome assessments were combined into a composite score of school‐age externalizing symptoms. A generalized procedure, developed from the correction for attenuation formula, is introduced to describe the relation of aggregation to predictive validity in longitudinal research.