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A longitudinal study is a correlational research study that involves repeated 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, it has been argued that they may 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.

Longitudinal studies allow social scientists to distinguish short from long-term phenomena, such as poverty. If the poverty rate is 10% at a point in time, this may mean that 10% of the population are always poor, or that the whole population experiences poverty for 10% of the time. It is not possible to conclude which of these possibilities is the case using one-off cross-sectional studies.

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 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.

• Case study
• Case series
• Cohort study
• Case control study
• Randomized database study

• Born in Bradford
• British Household Panel Survey
• Busselton Health Study
• Dunedin Longitudinal Study
• Framingham Heart Study
• German Socio-economic Panel Study
• Household, Income and Labour Dynamics in Australia Survey
• Luxembourg Income Study (LIS)
• Minnesota Twin Family Study
• National Longitudinal Survey of Youth
• Panel Study of Belgian Households
• Panel Study on Income Dynamics
• Seven Up!
• Wisconsin Longitudinal Study

• General Social Survey
• World Values Survey

• Time series
• Cross-sectional study
• Observational study

Longitudinal studies are beneficial in portraying the similarities and differences between several traits over several individuals over the course of several years, often decades.

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