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Introduction to Meta-Analysis Joseph Stevens, Ph.D., University of Oregon (541) 346-2445,...
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Transcript of Introduction to Meta-Analysis Joseph Stevens, Ph.D., University of Oregon (541) 346-2445,...
![Page 1: Introduction to Meta-Analysis Joseph Stevens, Ph.D., University of Oregon (541) 346-2445, stevensj@uoregon.edustevensj@uoregon.edu © Stevens 2006.](https://reader038.fdocuments.us/reader038/viewer/2022110322/56649d205503460f949f58e7/html5/thumbnails/1.jpg)
Introduction to Meta-Analysis
Joseph Stevens, Ph.D., University of Oregon(541) 346-2445, [email protected]© Stevens 2006
![Page 2: Introduction to Meta-Analysis Joseph Stevens, Ph.D., University of Oregon (541) 346-2445, stevensj@uoregon.edustevensj@uoregon.edu © Stevens 2006.](https://reader038.fdocuments.us/reader038/viewer/2022110322/56649d205503460f949f58e7/html5/thumbnails/2.jpg)
What is Meta-Analysis (MA)?
Term coined by Gene Glass in his 1976 AERA Presidential address
An alternative to the traditional literature review
Allows the reviewer to quantitatively combine and analyze the results from multiple studies
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What is Meta-Analysis (MA)?
Traditional literature review is based on the reviewer’s analysis and synthesis of study themes or conclusions
MA collects the essential empirical results from multiple studies and draw conclusions about the “overall” effect across studies no matter what the original study conclusions were
Thus a MA becomes a research study on research studies, hence the term "meta".
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Growth and Development of MA MA has developed substantially both in
methods and in applications (Larry Hedges, Ingram Olkin, John Hunter, and Frank Schmidt)
Literature review should be as systematic as primary research and study characteristics and design should provide a context for interpreting study results and conclusions (Glass)
MA now widely used in many disciplines (e.g., education, social sciences, medicine)
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Conducting a Meta-Analysis Researcher first collects studies on a
particular topic Information about studies is then
collated and coded Results of each study are translated into
a common metric, the study effect size Analysis is then conducted to
summarize effect size across studies or analyze relationships between covariates and effect size
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Effects of MA
An important consequence of the development of MA is the way it has changed our thinking about research Increased focus on a number of
important issues in science including publication biases
How to understand and summarize statistical results
Importance of effect size and statistical power
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Effect Size in MA Effect size makes meta-analysis possible
it is the “dependent variable” it standardizes findings across studies
such that they can be directly compared Any standardized index can be an “effect
size” (e.g., standardized mean difference, correlation coefficient, odds-ratio) as long as: It is comparable across studies It represents the magnitude and direction
of the relationship of interest It is independent of sample size
Different meta-analyses may use different effect size indices
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Which Studies to Review?
Should be as inclusive as possible Need to find all studies Include unpublished studies
Apples and Oranges A priori inclusion and exclusion
criteria Revision of criteria as MA proceeds More than one sample of studies for
different purposes
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Which Studies? Significant findings are more likely to be
published than nonsignificant findings (File drawer problem)
Critical to try to identify and retrieve all studies that meet your eligibility criteria
Potential sources for identification of documents computerized bibliographic databases authors working in the research domain conference programs dissertations review articles reference lists hand searching relevant journals government reports, bibliographies, clearinghouses
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Strengths of Meta-Analysis
Imposes a discipline on the process of summing up research findings
Represents findings in a more differentiated and sophisticated manner than conventional reviews
Capable of finding relationships across studies that are obscured in other approaches
Protects against over-interpreting differences across studies
Can handle a large numbers of studies (this would overwhelm traditional approaches to review)
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Weaknesses of Meta-Analysis
Requires a good deal of effort Mechanical aspects don’t lend themselves to
capturing more qualitative distinctions between studies
“Apples and oranges”; comparability of studies is often in the “eye of the beholder”
Most meta-analyses include “blemished” studies Selection bias posses continual threat
negative and null finding studies that you were unable to find
outcomes for which there were negative or null findings that were not reported
Analysis of between study differences is fundamentally correlational
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Examples of Different Types of Effect Sizes: Standardized Mean Difference
(continuous outcome) group contrast research
treatment groups naturally occurring groups
Odds-Ratio (dichotomous outcome) group contrast research
treatment groups naturally occurring groups
Correlation Coefficient association between variables research
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The Standardized Mean Difference
Represents a standardized group comparison on a continuous outcome measure.
Uses the pooled standard deviation (some situations use control group standard deviation).
Commonly called “Cohen’s d” or occasionally “Hedges’ g”.
pooleds
XXES 21
2
11
21
2221
21
nn
nsnsspooled
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The Correlation Coefficient
Represents the strength of association between two continuous measures.
Generally reported directly as “r” (the Pearson product moment coefficient).
rES
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Odds-Ratios
The Odds-Ratio is based on a 2 by 2 contingency table, such as the one below.
The Odds-Ratio is the odds of success in the treatment group relative to the odds of success in the control group.
Frequencies
Success Failure
Treatment Group a b
Control Group c d bc
adES
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Converting results into a common metric Can convert p-values t, F, etc.
into the standardized effect size metric being used in the meta-analysis (e.g., d, r)
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Interpreting Effect Size Results Cohen’s “Rules-of-Thumb”
standardized mean difference effect size small = 0.20 medium = 0.50 large = 0.80
correlation coefficient small = 0.10 medium = 0.25 large = 0.40
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Interpreting Effect Size Results Rules-of-thumb do not take into
account the context of the intervention a “small” effect may be highly meaningful
for an intervention that requires few resources and imposes little on the participants
small effects may be more meaningful for serious and fairly intractable problems
Cohen’s rules-of-thumb do, however, correspond to the distribution of effects across meta-analyses found by Lipsey and Wilson (1993)
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Interpreting Effect Size Results Findings must be interpreted within
the bounds of the methodological quality of the research base synthesized.
Studies often cannot simply be grouped into “good” and “bad” studies.
Some methodological weaknesses may bias the overall findings, others may merely add “noise” to the distribution.
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Traditional Narrative reviews Vote-counting