Abstract
It is explained, both by considering earlier theoretical discussions as well as by providing analytic proof, why it is important to base analyses of developmental processes on intra-individual variation (replicated time series analysis) instead of on inter-individual variation. It is often found that replicated time series analysis yields different (person-specific) dynamic model structures across replications. A new powerful approach is presented that enables the identification of a valid common (nomothetic) dynamic model if individual replications yield person-specific dynamic model structures.