Notes
Slide Show
Outline
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Step 3:
  • Critically Appraising the Evidence:
  • Statistics for Harm and Etiology
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Table of Contents
  • Clinical Statistics Calculator (Excel)
  • Statistics for:
    • Harm/Etiology & Prognosis
      • Control Event Rate (CER) & Experimental Event Rate (EER)
      • Number Needed to Harm (NNH)
        • Absolute Risk Increase (ARI)
      • Relative Risk (RR)
      • Odds
      • Odds Ratio (OR)
  • Practice Exercises
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Making It Easier
  • If available, find the best evidence in secondary sources where analysis has already occurred.
  • If not pre-assessed, use critical appraisal worksheets to help you through the process.
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Importance of Critically
Appraising the Evidence
  • Understanding the Limitations of the Author’s Analyses and Interpretations of the Data
  • Assessing Internal Validity
  • Assessing External Validity
  • Identifying Potential Confounding Variables
    • Simpson’s Paradox
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Critical Appraisal Basics
  • View movie as:
    • QuickTime (.mov)
    • Flash (.swf)






  • Double-click on video for full-screen mode.
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Generalized 2x2 Clinical Table
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Control Event Rate (CER) and
 Experimental Event Rate (EER)
  • Experimental Event Rate (EER)
    • The proportion of patients (in the intervention) who experienced the target  outcome
  • Control Event Rate (CER)
    • The proportion of patients (in the comparison group) who experienced the target outcome
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Calculating CER and EER
  • Experimental Event Rate (EER)
    • a/(a+b)


  • Control Event Rate (CER)
    • c/(c+d)
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Number Needed to Harm (NNH)
  • The estimated number of people treated for each patient the treatment harms beyond baseline/control risk.
  • So smaller numbers indicate greater probability of harm as a result of the treatment
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Calculating NNH
  • The calculation of NNH is the same as that of NNT, but the interpretation is differentiated by the adverse outcomes present in harm studies. Here the term NNH effectively replaces NNT.
  • Similarly Absolute Risk Increase (ARI) is calculated in the same way that ARR is calculated but the intervention  increases the amount of harm instead of therapeutic gain.
  • NNH = 1/ARI = 1/|CER – EER|
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NNH Video
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Relative Risk (RR)
  • The number of treated/exposed patients with the target outcome for every patients in the control with the target outcome
    • (Also used in therapy articles)

  • RR = EER / CER
  •          = (a/(a+b)) / (c/(c+d))
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Relative Risk Video
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Odds
  • The number of times the target outcome occurred in patients exposed to the risk for each time the target outcome occurred in patients not exposed to the risk.
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Odds Ratio (OR)
  • OR = (a/b) / (c/d)
  •       = a*d / b*c


  • A measure of association
  • When large, there is greater association
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Try it on your own.
  • Critical Appraisal Practice Exercises
    • From CEBM
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Links to Other Websites
and Hands-On Activities
  • EBM Glossary
    • From CEBM
  • Critical Appraisal Practice Exercises
    • From CEBM
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Congratulations!
You have successfully completed Step 3.
The End