The article presents a method for interpreting longitudinal data based on analysis of changes in the distribution of the phenotype in a population. The histogram is one of the principal characteristics being analyzed. Markov's process with discrete states and continuous time is associated with evolution of each histogram. The method has been used to analyze data on a longitudinal study of the intellectual development quotient enabling the investigators to make a number of conclusions concerning development of Russian children in the preschool and school periods and also on the correlation of the combined effect of systemic factors and the level of intellectual development.