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A longitudinal study is a correlational research study that involves observations of the same items over long periods of time, often many decades. Longitudinal studies are often used in psychology to study developmental trends across the life span. The reason for this is that unlike cross-sectional studies, longitudinal studies track the same people, and therefore the differences observed in those people are less likely to be the result of cultural differences across generations. Longitudinal studies are also used in medicine to uncover predictors of certain diseases.
Because longitudinal studies are observational, in the sense that they observe the state of the world without manipulating it, they have less power to detect causal relationships than do experiments. But because of the repeated observation at the individual level, they have more power than cross-sectional observational studies, by virtue of being able to exclude time-invariant unobserved individual differences, and by virtue of observing the temporal order of events.
Types of longitudinal studies include cohort studies and panel studies. Cohort studies sample a cohort, defined as a group experiencing some event (typically birth) in a selected time period, and studying them at intervals through time. Panel studies sample a cross-section, and survey it at (usually regular) intervals.
A prospective longitudinal study sets out to test particular hypotheses through analysis of subsequently collected data.
A retrospective study is a longitudinal study that looks back in time. For instance a researcher may look up the medical records of previous years to look for a trend.
- 1970 British Cohort Study (BCS70)
- Avon Longitudinal Study of Parents and Children (ALSPAC)
- Born in Bradford
- British Household Panel Survey
- Busselton Health Study
- Canadian Longitudinal Study on Aging / Étude longitudinale canadienne sur le vieillissement (CLSA-ÉLCV)
- Child Development Project
- Children of Immigrants Longitudinal Study (CILS)
- Dunedin Longitudinal Study
- Dunedin Multidisciplinary Health and Development Study
- Fragile Families and Child Wellbeing Study
- Framingham Heart Study
- Genetic Studies of Genius Terman' study is the longest running in the world.
- Grant Study
- German Socio-Economic_Panel (SOEP)
- Health and Retirement Study
- Household, Income and Labour Dynamics in Australia Survey
- Human speechome project
- Lothian birth-cohort studies
- Luxembourg Income Study (LIS)
- Madras longitudinal study of schizophrenia
- Millennium Cohort Study (MCS)
- Millennium Cohort Study (United States)
- Minnesota Twin Family Study
- National Child Development Study (NCDS)
- National Longitudinal Study of Children and Youth (NLSCY)
- National Children's Study
- National Longitudinal Survey of Youth
- Pacific Islands Families Study
- Panel Study of Belgian Households
- Rotterdam Study
- Seattle Longitudinal Study
- Wisconsin Longitudinal Study 
- UK Adult Psychiatric Morbidity Survey
- UK National Child Developmental Study
- WHO's Study on global AGEing and adult health (SAGE) 
Repeated Cross-Sectional Surveys
References & Bibliography
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