- (written as Longitudinal Study)Any study using time series data. In econometric studies, the object is often to analyse the determinants of (the growth of) income, expenditure (for example, national health care expenditures) or consumption. Increasingly there are micro datasets available as well as macro datasets. Clinical trials, when individual people are followed, are called longitudinal cohort studies or follow-up studies (the two terms are substitutable). If individual people are not followed, but classes of people (usually age classes) are restudied, one has a longitudinal cross-sectional study.