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Systematic Reviews

Hierarchy of Evidence & Study Types

Scientific evidence is not created equal. The hierarchy of evidence is represented by the evidence pyramid.  The study with the highest level of evidence, i.e., systematic review or meta-analysis is at the top of the pyramid, followed by randomized controlled trials (RCTs), observational studies and the lowest level evidence being case reports, case-series, or expert opinions residing at the bottom of the pyramid.  

Greenhalgh T. (2010). How to read a paper: the basics of evidence-based medicine (4th ed.). Wiley-Blackwell.

The following table shows an example of the study design that best answered the different types of commonly seen clinical questions.


Fineout-Overholt, E., & Johnston, L. (2005). Teaching EBP: asking searchable, answerable clinical questions. Worldviews Evid Based Nurs, 2(3), 157-160.

"The choice of study type will mainly depend on the research question being asked." The article on "What types of studies are there?" gives a brief description to the different study designs.  

Adapted from [Internet]. Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG); 2006-. What types of studies are there? 2016 Jun 15 [Updated 2016 Sep 8]. Available from:

Randomized controlled trials (RCTs)

RCTs provide the best results when trying to find out if there is a cause-and-effect relationship. RCTs provide the most reliable answers for treatment/therapy/intervention/diagnostic test where they can how what happens if you opt to not have the treatment or diagnostic test or compared to other treatments or diagnostic tests.

In order to be able to reliably assess how effective the treatment is, the following things also need to be determined before the study is started:

  • How long the study should last

  • How many participants are needed

  • How the effect of the treatment should be measured

Cohort studies

Best to answer prognosis, harm and causation type of questions as a group of people who are observed frequently over a period of many years. As it might NOT be ethical to conduct a RCT to find out if smoking causes cancer. Therefore, using cohort studies, the researchers then observe how the health of the people in both groups (smoke and non-smoke) develops over the course of several years, whether they become ill, and how many of them pass away. Cohort studies can have a prospective (forward-looking) design or a retrospective (backward-looking) design.

Cohort studies are especially useful if you want to find out how common a medical condition is and which factors increase the risk of developing it. They can answer questions such as:

  • How does high blood pressure affect heart health?

  • Does smoking increase your risk of lung cancer

Case-control studies

Second best to answer harm/causation/prognosis type of questions as it is not as expensive or time-consuming as RCTs or cohort studies. But it is often difficult to tell which people are the most similar to each other and should therefore be compared with each other. Usually data gathered based on past events which are dependent on the participants’ memories. 

Still, case-control studies can help to investigate the causes of a specific disease, and answer questions like these:

  • Do HPV infections increase the risk of cervical cancer?

  • Is the risk of sudden infant death syndrome (“cot death”) increased by parents smoking at home?

Cross-sectional studies

Use to answer screening/diagnosis type of question. The classic type of cross-sectional study is the survey: A representative group of people – usually a random sample – are interviewed or examined in order to find out their opinions or facts. Unlike cohort studies, cross-sectional studies can be relatively quick and inexpensive as data is collected only once.

Cross-sectional studies can answer questions such as these:

  • How tall are German men and women at age 20?

  • How many people have cancer screening?

Qualitative Studies

Unlike other kinds of research, qualitative research does not rely on numbers and data. Instead, it is based on information collected by talking to people who have a particular medical condition and people close to them. Make use of a number of methods such as interviews, diaries, observation, focus groups (both textual and visual analysis) to interpret the findings.

Qualitative studies can answer questions such as these:

  • How do women experience a Cesarean section?

  • What aspects of treatment are especially important to men who have prostate cancer?