Meta analyses
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A research group talk about meta-analyses

A research group talk about meta-analyses

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Meta analyses Presentation Transcript

  • 1. Meta-Analysis: An Introduction Bob O'Hara
  • 2. Useful References Harrison, F. (2011) Getting started with meta-analysis. Methods in Ecology and Evolution 2 : 1–10 http://dx.doi.org/10.1111/j.2041-210X.2010.00056.x Podcast: http://bdown.astream.com/mee/fharrison.mp3 Stewart, G. (2010) Meta-analyses in applied ecology. Biol. Letters 6 : 78-81 http://dx.doi.org/10.1111/10.1098/rsbl.2009.0546
  • 3. The Problem: lots of studies of the same thing
  • 4. We want to combine the studies “ Evidence synthesis”
  • 5. Meta-analyses let us do this
  • 6. “Meta-analysis” introduced in 1976 Glass, G.V. (1976) Primary, Secondary, and Meta-Analysis of Research. Educational Researcher 5 : 3-8. http://dx.doi.org/10.3102/0013189X005010003
  • 7. ...but the idea is much older Simpson, R.J.S. & Pearson, K. (1904) Report On Certain Enteric Fever Inoculation Statistics. The British Medical Journal 2 : 1243-1246 http://www.jstor.org/stable/20282622
  • 8. Popular in Education and Medicine
  • 9. ...and now being used in biology
      Number of applied meta-analysis articles in ISI journals in Ecology and Evolution
      Stewart G Biol. Lett. 2010;6:78-81
      ©2010 by The Royal Society
  • 10. What It Does
    • Combines studies' estimates of an effect
      • e.g. the effect of a drug, from several clinical trials
  • 11. How to do it In 3 ½ steps
  • 12. Step 1: Get your data
    • Search the literature
    • 13. Use criteria for deciding which studies should be included
      • Must be clear here: want to avoid bias
  • 14. Step 2: Chose a statistic
    • Effect size
    • 15. Have to interpret on the same scale
    • 16. Standardised
      • Cohen's d , Hedges' g , Odds ratio, etc
  • 17. Step 3: Calculate & present the stats
    • Forest Plots
  • 18. Step 3a: Publication bias?
    • Funnel Plots
    “ File drawer problem”
  • 19. Heterogeneity Across Studies
    • Studies often measure slightly different things
      • Same statistic, different species
    • Means extra variation
      • Use random effects models
  • 20. Meta-regressions
    • Use properties of study to explain responses
      • e.g. does body size affect the statistic?
    • In essence a weighted regression
    Has potential to become very complicated
  • 21. Finally: write it up
  • 22. Finally: write it up ...and hide it in your bottom drawer