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1. Randomised Controlled Trial: a group of patients are signed up to help answer a question, and somehow they are randomly selected to get one treatment or another. This can be a choice between two or more different medications or surgery options, and may or may not include a placebo. This sort of trial is the most difficult to perform but the results are usually the best for seeing how good some treatment really is. The information is collected up front and on purpose in an organized way.
2. Cohort studies: two groups (each group is a “cohorts”) differ in exposure to the item being studied, and they are compared to each other to learn what happens when that item is changed. One good example is people who do and people who do not drink alcohol. Another is smokers and non-smokers. It is much easier to do a cohort study because the groups have self-selected themselves out over years and the data can be found by chart reviews without a lot of up-front planning. Sometimes the data comes from huge databases collected by the government.
3. Case-controlled studies: patients with a given condition are matched with another group that does not have that condition, and their differences are studied. These are also often gathered from databases and chart reviews to assess trends in large groups of people
4. Cross-sectional surveys: patients are given questionnaires and asked to rate things under consideration. Sometimes they are surveyed twice – one a the beginning of the study and again at the end. These studies help figure out how people change, but a bad questionnaire gives a bad answer. And since people are answering by their memory of things sometimes, the results can be imperfect.
5. Case reports: an interesting thing has happened, and the doctor has submitted the case for the journal to report. Sometimes these report a series of cases that are similar.