Groups of people are selected on the basis of their exposure to a particular agent and followed up for specific outcomes. 5 Cross sectional surveys Survey or interview of a sample of the population of interest at one point in time 6 Case reports. A consensus of experience from the good and the great. Something a bloke told you after a meeting or in the bar. You may note that this ranking has an evolutionary order, moving from simple observational methods at the bottom through to increasingly sophisticated and statistically refined methodologies.
Social phenomena are even more complex and difficult to isolate.
Many biomedical discoveries have had to await the development of techniques capable of dealing with populations in a statistical manner. Such discoveries were often empirical, and pre-dated the theories that would back them up. Techniques which are lower down the ranking are not always superfluous. Here, you need a cohort, where you find a group exposed to the agent by chance or their own choice and compare how they fare with another group who were not exposed. The future may also see changes in the hierarchy of scientific evidence.