Hepatitis B and Liver Cancer
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Slide 1 :
Evidence-Based Medicine Ambulatory Care Block 2008-2009 Study Design: Systematic Reviews & Meta Analyses John H. Choe, MD, MPH Jenny Wright, MD Scott Steiger, MD Sherry Dodson, MLS
Slide 2 :
Topics Recap from previous week Introduction to randomized controlled trials Statistical measures of excess risk Introduction to systematic reviews and meta-analyses Two examples from the literature Assessing validity, importance, and applicability Brief overview of case-control and cohort studies
Slide 3 :
Steps to Practicing EBM Convert the need for information into an answerable question. Track down the best evidence with which to answer that question. Critically appraise the evidence for its validity, impact, and applicability. Share this evidence with our colleagues and our patients. 5. Integrate the evidence with our clinical expertise and our patient’s characteristics and values.
Slide 4 :
Evidence Pyramid pyramid modified from: Navigating the Maze, University of Virginia, Health Sciences Library
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Judging Studies VALIDITY IMPORTANCE APPLICABILITY
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Measuring Differences Analyzing differences between groups Count data: chi-squared (?2) test Continuous variables: Testing differences between means 2 groups: t-test = 2 groups: ANOVA (ANalysis Of Variance) Statistically significant when p<0.05 P<0.05 = less than a 5% probability that the difference between groups was from chance rather than “real” Equivalent concept if 95% CI does not include 1
Slide 8 :
Terms to Describe Excess Risk Relative Risk Relative Risk Reduction Absolute Risk Reduction (= Risk Difference= Attributable Risk= Absolute Diff.) Drop in percent risk for disease due to intervention = (rate in exposed–rate in nonexposed) NNT / NNS / NNH: No. needed to Treat/Screen/Harm No. of patients who must be treated / screened to prevent 1 disease outcome (e.g. cancer or death) 1 / (rate in untreated group – rate in treated group) 1 / (Absolute Risk Reduction)
Slide 9 :
Math: Calculating Excess Risk Imagine that 20% of placebo group get disease, compared to 15% of drug group– Risk Difference (Absolute Risk Reduction)? 0.20 – 0.15 = 0.05 Relative Risk is? 0.15 / 0.20 = 0.75 Relative Risk Reduction is? (1 –RR) = (1 –0.75) = 0.25, or 25% Number needed to treat to prevent 1 event? 1 / (ARR) = 1/(0.05) = 20
Slide 10 :
Strauss et al Math Equivalent ? Equally Important
Slide 11 :
Putting it all together: RCTs Does this RCT apply to our specific question? Are we satisfied with the randomization and blinding of participants (and study staff)? Was follow-up reasonably complete, and can we account for all enrolled participants and their outcomes? Intention to treat analysis? Was there a statistically significant difference favoring a treatment/ intervention/ screening? Were these results clinically significant and applicable to my patient and my problem?
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Slide 13 :
Systematic Review Defined: Summary of the medical literature that uses explicit methods to systematically perform a comprehensive literature search, critically appraise, and synthesize the world literature on a specific topic Investigators do not recruit individual subjects, but instead perform a comprehensive literature search to find relevant articles– An investigation whose “subjects” are other studies Contrast: Traditional literature reviews usually do not try to synthesize study results or are not always comprehensive Clear clinical question is still the key to a well-designed systematic review! Although many systematic reviews will try to summarize results from the included reviewed studies, not all will try to combine results statistically
Slide 14 :
Meta-Analysis Defined: A systematic review that uses quantitative statistical methods to synthesize and summarize the results. Just as in other systematic reviews: A meta-analysis has other studies as “subjects”, and finding other studies must be done comprehensively Goals are to minimize bias and random error while potentially increasing study power by considering a wide range of studies on a particular question
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