Definition Longitudinal study
A longitudinal study is the process of data collection that is performed recurrently over time on the same statistical unit. Longitudinal data is primarily used to examine time series and to generate forecasts based on that data.
Examples of longitudinal studies include the annual measurement of a country's GDP and the daily measurement of the temperature at Mount McKinley.
Surveys lead to longitudinal when the exact same people are recurrently interviewed in a panel on the exact same topic. The opposite of longitudinal data are cross-sectional data.
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