Faculty of Health and Medical Sciences

Appraise the evidence

Study type: Aetiology/harm

Learn more about:


Validity

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Clinical importance

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Applicability

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EBP calculations

Adverse outcome Present (case) Absent (control)  
Exposed Yes (Cohort) a b a + b
  a + c b + d a + b + c + d

RCT, cohort study: Relative risk (RR) = [a/(a+b)]/[c/(c+d)]

Case-control study: Odds ratio (OR) = ad/bc

Calculations (2) for studies of aetiology/harm

Number needed to Harm (NNH)

Individualise for your patient by estimating their Event Rate for the adverse event if they were not exposed to the causative factor (PEER = Patient’s Expected Event Rate)

NNH = PEER (OR – 1) + 1
      PEER (OR – 1) x (1 – PEER)

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Question of harm/aetiology:
Basic critical appraisal
Advanced critical appraisal