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. In probability theory and statistics, correlation, also called correlation coefficient, is a numeric measure of the strength of linear relationship between two random variables. ... Cross-sectional studies form a class of research methods that involve observation of some subset of a population of items all at the same time. ...

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. An observational science is a science where it is not possible to construct controlled experiments in the area under study. ... The philosophical concept of causality, the principles of causes, or causation, the working of causes, refers to the set of all particular causal or cause-and-effect relations. ... hello can you hear me For the card game, see Experiment (game). ...

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. It has been suggested that this article or section be merged with Cohort (statistics). ... It has been suggested that this article or section be merged with Cohort (statistics). ... For other meanings see cohort In statistics and demography, a cohort is a group of subjects — most often humans from a given population — defined by a condition on their date of birth. ...

Examples

• Dunedin Longitudinal Study
• Framingham Heart Study
• Minnesota Twin Family Study
• World Values Survey (repeated cross-sectional)
• Panel Study on Income Dynamics
• German Socio-economic Panel Study
• British Household Panel Survey
• Seven Up!
• Household, Income and Labour Dynamics in Australia Survey
• Luxembourg Income Study (LIS)