Meta-Analysis: An Introduction
Bob O'Hara
Useful References
Harrison, F. (2011) Getting started with meta-analysis. Methods
in Ecology and Evolution 2:
110http://dx.doi.org/10.1111/j.2041-210X.2010.00056.xPodcast:
http://bdown.astream.com/mee/fharrison.mp3
Stewart, G. (2010) Meta-analyses in applied ecology. Biol.
Letters 6:
78-81http://dx.doi.org/10.1111/10.1098/rsbl.2009.0546
The Problem: lots of studies of the same thing
We want to combine the studies
Evidencesynthesis
Meta-analyses let us do this
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
...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: 1243-1246http://www.jstor.org/stable/20282622
Popular in Education and Medicine
...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
What It Does
Combines studies' estimates of an effecte.g. the effect of a
drug, from several clinical trials
How to do it
In 3 steps
Step 1: Get your data
Search the literature
Use criteria for deciding which studies should be includedMust
be clear here: want to avoid bias
Step 2: Chose a statistic
Effect size
Have to interpret on the same scale
StandardisedCohen's d, Hedges' g, Odds ratio, etc
Step 3: Calculate & present the stats
Forest Plots
Step 3a: Publication bias?
Funnel Plots
File drawer problem
Heterogeneity Across Studies
Studies often measure slightly different thingsSame statistic,
different species
Means extra variationUse random effects models
Meta-regressions
Use properties of study to explain responsese.g. does body size
affect the statistic?
In essence a weighted regression
Has potential to become very complicated
Finally: write it up
Finally: write it up
...and hide it in your bottom drawer
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