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Systematic Reviews for Health Sciences and Medicine

An introduction to systematic reviews, with examples from health sciences and medicine

Evidence Based Practice

It's important to ask a question that can be answered through use of evidence rather than subjective judgment. Evidence-based medicine (EBM) - like evidence-based practice in any discipline - draws on empirical evidence. This may include laboratory studiesclinical trials, and real-world prospective or retrospective studies. You may need to rephrase the question according to whether the evidence base is in diagnosis, therapy & interventions, prognosis, or assessments of harm.

EBM gives more weight and authority to randomised controlled clinical trials and cohort studies than case studies. It gives more weight again to meta-analyses synthesising the results of many RCTs. 

Figure 1: The hierarchy of evidence



Greenhalgh (1997) gives an alternative ranking of the types of primary study :
     (1) Systematic reviews and meta-analyses
     (2) Randomised controlled trials with definitive results (confidence intervals that do not overlap the threshold
     clinically significant effect)
     (3) Randomised controlled trials with non-definitive results (a point estimate that suggests a clinically
     significant effect but with confidence intervals overlapping the threshold for this effect)
     (4) Cohort studies
     (5) Case-control studies
     (6) Cross sectional surveys
     (7) Case reports.


Further Reading

Greenhalgh, Trisha (1997). How to read a paper : getting your bearings (deciding what the paper is about) BMJ 1997; 315 :243 PDF Link



It's important to ask a question in enough detail to get relevant and appropriate results.

You may have a specific research question to address. A concept map to explore key terms likely to feature in your target literature - with synonyms and related terms - is often helpful.

Or you may be dealing with one or more patients in clinical casework. A clinical question has a number of essential facets, outlined in the PICO(TS) mnemonic. 


An alternative search strategy tool for qualitative/mixed methods research has been suggested: the SPIDER mnemonic.