"Nothing is true without its opposition." (Martin Walser)
Another study type which may or may not be a randomized controlled trial (RCT) are the so called longitudinal trials. 'Longitudinal' refers to a time aspect and "direction of view" Prospective means forward looking, retrospective means looking back. Typically these are cohort studies or case-control studies, to compare groups. In the spefic case of a case-control study, attention is paid to the fact that the group in terms of relevant other factors (age occupation, etc.) do not differ significantly. The strength of this design is to identify risks.
Example for a retrospective Case Control study