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Literature Searching

Follow our step-by-step guide to help you carry out a good literature search

What is critical appraisal?

"Critical appraisal is the process of systematically examining research to judge its trustworthiness, value and relevance in a particular context." (Burls, 2009, p. 1)

Introduction to critical appraisal

Basics of critical appraisal

critical appraisal of evidence

Types of evidence

 The levels of evidence pyramid provides a way to visualize both the quality of evidence and the amount of evidence available. For example, systematic reviews are at the top of the pyramid, meaning they are both the highest level of evidence and the least common. As you go down the pyramid, the amount of evidence will increase as the quality of the evidence decreases.

Levels of evidence pyramid with coloured layers and corresponding text

1, 2, 3 : Filtered resources (the quality of studies are appraised and often make recommendations for practice e.g. guidelines)

4, 5, 6 : Unfiltered resources (you review what you find to make sure it is valid and reliable)

7 : Background information, expert opinion (not necessarily backed by research studies)


Systematic reviews v primary studies

Systematic Reviews and Meta Analyses

Systematic reviews can often provide the best evidence. They research findings in thorough systematic literature searches.  But:

  • The process of a rigorous systematic review can take years to complete and findings can therefore be superseded by more recent evidence.
  • The methodological rigor and strength of findings must be appraised by the reader before being applied in clinical settings.
  • A large, well-conducted RCT may provide more convincing evidence than a systematic review of smaller RCTs

Primary Studies

If a current systematic review isn't available, go to primary studies e.g. papers directly from the researcher who conducted the study. The table below lists optimal study methodologies for the main types of questions.

Question Best Research Design for Question
Therapy (Treatment) Randomised Controlled Trial (RCT)
Prevention RCT or Prospective Study
Diagnosis RCT or Cohort Study
Prognosis (Forecast) Cohort Study and/or Case-Control Series
Etiology (Causation) Cohort Study
Meaning Qualitative Study


Note that the Clinical Queries filter available in  CINAHL/Medline matches the question type to studies with appropriate research designs. 

When searching primary literature, look first for reports of clinical trials that used the best research designs. Remember as you search, though, that the best available evidence may not come from the optimal study type. For example, if treatment effects found in well designed cohort studies are sufficiently large and consistent, those cohort studies may provide more convincing evidence than the findings of a weaker RCT.

Grey literature & ongoing research

CATs: Appraising the evidence

A cat mnemonic for Critical Appraisal Tools (CATs)

Use these Critical Appraisal Tools (CATs) to help you evaluate the quality of particular types of evidence

Critical Appraisal bite-size series of videos