Definition: Longitudinal research is an observational technique that involves studying the same group of individuals over an extended period of time. Data is first collected at the outset of the study, and may then be gathered repeatedly throughout the length of the study. In some cases, longitudinal studies can last several decades. Read more...
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How can some one study the same group of individual
Use quasy experiment or correlation study measure two variables of interest.
longitudinal studies helps to investigate the L2 = L1 hypothesis