PEPs - Clinical Quality and Evidence

Module 2 - Evidence

Types of evidence

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.  

Attachment: Classifying study design for questions of effectiveness.pdf

Algorithm for classifying study design for questions of effectiveness - Adapted from NICE:


At this point, you may wish to look back at the teaching materials from Evidence Based Medicine in Phase 1 to remind yourself about the different types of trial. These materials are available within Blackboard.

There’s also a very good account of the types of clinical study in an online article:

How to read a paper: getting your bearings (deciding what the paper is about) - Greenhalgh, T. (1997)