Epidemiological studies
These are often the studies behind the big pronouncements we hear about on the news. The findings of a study that says eating chocolate improves mood are likely to be based on a questionnaire or other means of gathering information in which people's mood and their chocolate consumption are measured. Epidemiological studies are not performed in a laboratory setting; they usually take place in the real world and involve large numbers of people.

Epidemiological studies might point to a plausible cause and effect, but it usually takes an experiment or clinical study to provide real proof.