The systematic study of constancy and change across the life-span (i.e., in inter-individual differences in intra-individual change). Strictly speaking, with a major commitment to the study of intra-individual change, it is based on a longitudinal, not cross-sectional, designs, or some combination of the two as a means of accounting for learning effects derived from repeated observations. Moreover, developmental research is not necessarily carried out over ontogenetic time (e.g., days, months, years), but also by means of microgenetic studies consisting of much shorter real-time scales. (e.g., microseconds, seconds, minutes).
See Cross-sectional design, Developmental (or ontogenetic) time, Developmental trajectories, Intra-individual differences, Life course analysis, Longitudinal design, Microgenetic method, Life course analysis