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The key difference between experimental and observational study is that an experimental study is a study where the researcher has control over most of the variables. On the other hand, an observation study is a study where the researcher merely observes the subject without controlling any variables.
A longitudinal study, like a cross-sectional one, is observational. So, once again, researchers do not interfere with their subjects. However, in a longitudinal study, researchers conduct several observations of the same subjects over a period of time, sometimes lasting many years.