A longitudinal study observes the same individuals repeatedly. Within-person changes distinguish temporal development from persistent differences between people, although calendar-period changes, selective dropout and measurement error still require attention. A random-intercept linear mixed model can represent persistent individual differences and correlated repeated measurements.
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A longitudinal study is a research design that involves repeated observations of the same variables (such as individuals, groups, or phenomena) over an extended period of time, which can range from months to many years or even decades. Longitudinal studies are often used in various fields, including psychology, sociology, medicine, and education, to track changes and developments, identify trends, and examine causal relationships.