A prospective study is one that identifies a scientific (usually medical) problem to be studied, specifies a study design protocol (e.g. what you’re measuring, who you’re measuring, how many subjects, etc.), and then gathers data in the future in accordance with the design. The definition of the problem under study does not change once the data collection starts.
A retrospective study is one in which you look backwards at data that have already been collected or generated, to answer a scientific (usually medical) problem.
Prospective studies are generally regarded as cleaner and more reliable, due to several shortcomings of retrospective studies:
- Since the data are already available, the question to be answered can be influenced by the data
- Similarly, the exact data (or subset of data) used to answer the question can be drawn in a way that produces the desired answer, or a more noteworthy or interesting answer
- There is a temptation to modify the question being studied as the data are examined