Medical information can come from a variety of sources:
word of mouth (family, friends, GP)
media (newspapers, TV, internet)
published reports in medical and scientific journals
To make decisions about the care of patients, the information used by doctors should come from the medical or scientific literature. But this evidence arises from several different study designs.
If we are trying to establish if a treatment is effective or not, the two main types of evidence we can collect are observational studies and experimental studies. Observational studies collect information about patients without trying to influence their exposure to a treatment or to something that might cause or prevent disease.
Experimental studies assign the exposure or intervention to individuals or groups.
The attached document below outlines the different types of each form of study.
Note that case reports (single case) and case series (several cases) can also be considered as a form of evidence: they are evidence that two things could be linked. Further investigation is needed to investigate whether or not this is the case.