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Evidence Based Medicine (EBM)

This guide is designed to walk you through the Evidence Based Medicine process: the elements of a well-formulated clinical question, types of studies, and the key critical appraisal questions that help determine the validity of evidence.

Different study designs

Meta-analysis: A statistical technique that summarizes the results of several studies in a single weighted estimate, in which more weight is given to results of studies with more events and sometimes to studies of higher quality.


Systematic review:
 a review in which specified and appropriate methods have been used to identify, appraise, and summarise studies addressing a defined question. It can, but need not, involve meta-analysis. 
 

Further reading: 

  1. Akobeng AK. Understanding systematic reviews and meta-analysisArchives of Disease in Childhood. 2005 Aug; 90(8): 845–848.
  2. Hemingway P, Brereton N. What is a systematic review? 2nd ed. [Internet]. Newmarket, UK: Hayward Medical Communications; 2009 [cited 2015 Apr 30]. 

 


Randomised controlled trial:
an experimental design where participants are randomly assigned to two or more groups: at least one (the experimental group) receiving an intervention that is being tested and another (the comparison or control group) receiving an alternative treatment or placebo. This design allows assessment of the relative effects of interventions. This design is good for answering questions such as:

  • Is this intervention effective?
  • Is intervention x more effective than intervention y in treating patients with similar characteristics to my patient?

Controlled (non-randomised) clinical trial: a trial in which participants are assigned to two or more different treatment groups. Group assignment is by a method other than random allocation. Non-randomised controlled trials are more likely to suffer from bias than RCTs.

Further reading: Coggon D, Rose G, Barker DJP. Experimental studies. In: Epidemiology for the uninitiated. [Internet]. London: BMJ (British Medical Journal) Publishing; 1997. [Cited 2015 Apr 30].


Cohort study: an observational (i.e. non-experimental) study design that follows a group of people (a cohort), and then looks at how events differ among people within the group. A study that examines a cohort, which differs in respect to exposure to some suspected risk factor (e.g. smoking), is useful for trying to ascertain whether exposure is likely to cause specified events (e.g. lung cancer). Prospective cohort studies (which track participants forward in time) are more reliable than retrospective cohort studies. Cohort studies are useful in answering questions such as:

  • What risk factors predict disease?
  • What factors cause these outcomes?
  • What happens with this disease over time?
  • What happens to the patient if a diagnostic test is positive?

  Cohort studies: a brief overview (5:33 mins, Prof T. Shaneyfelt, University of Alabama at Birmingham)

Case-control study: an observational (i.e. non-experimental) study design that matches a group of people who have experienced an event (usually an adverse event) with a group of people who have not experienced the same event. The study then looks at how exposure to risk factors differed between the two groups. This type of study design is most useful for trying to ascertain:

  • What risk factors predict disease?
  • What happens with this disease over time?
  • Does this new diagnostic test (the index test) perform as well as the old 'gold' (or reference) standard?

  Cohort and case control studies (4:22 mins, Dr G. Martin, Global Health) 

Cross-sectional study: This observational (i.e. non-experimental) study measures the prevalence of health outcomes, the determinants of health, or both, in a population at a particular point in time or over a short period. This design is good for answering questions such as:

  • How prevalent is the outcome, disease, or risk factor in a population?
  • What risk factors are associated with these outcomes?
  • Does this new diagnostic test (the index test) perform as well as the old 'gold' (or reference) standard?
     
Further reading: Coggon D, Rose G, Barker DJP. Case-control and cross-sectional studies. In: Epidemiology for the uninitiated. [Internet]. London: BMJ (British Medical Journal) Publishing; 1997. [Cited 2015 Apr 30].


Case series:
A descriptive, non-comparative study that analyses a group of people who have a disease (usually rare) or who have been exposed to a risk factor or intervention. This design is considered 'hypothesis-generating' in that a case series usually gives rise to questions that need a higher level of study design to answer. Good for answering the question: should we research this topic further?

Glossaries