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► Have you ever wanted to learn about meta-analysis or conduct a meta-analysis but didn't know where to start? This webpage is devoted to providing you expert opinion on what you need to know to start your own meta-analysis.
► With the thousands of meta-analyses conducted in all areas of psychology over the past few decades, there has been an ever-increasing number of articles, books, and software programs devoted to how to conduct meta-analyses. Below, experts on meta-analysis provide their suggesstions on which which of the many sources of information are the most useful and why -- so that the user has an easy-to-use starting place for learning everything about meta-analyses.
Where should I start?
If you want to learn what is a meta-analysis...
If you want to learn how to start conducting a meta...
What is a meta-analysis?
A meta-analysis statistically combines the results of several studies that address a shared research hypotheses.
Just as individual studies summarize data collected from many individual participants in order to answer a specific research question (e.g., each participant is a separate data point), a meta-analysis summarizes data from individual studies that concern a specific research question (e.g., each data point is each individual study).
The results of each individual study are converted to a standardized effect size. A meta-analysis combines...
Three Basic Questions
- A meta-analysis answers three general questions:
- Central tendency – The central purpose of a meta-analysis is to test the relationship between two variables such that X causes Y. Central tendency refers to identifying whether X affects Y via statistically summarizing signficance levels, effect sizes, and/or confidence intervals. You are trying to answer whether X affects Y, is the effect significant, and how strong is that effect?
- Variability – There is always going to be some degree of variation between the outcomes of the individual studies that compose the meta-analysis. The question is whether the degree of variablity is signficantly different than what we would expect by chance alone. If so, then its called heterogeneity.
- Prediction – If there is heterogeneity (variability), then we look for moderating variables that explain the variability. In other words, does the effect of X on Y differ with moderator variables?
Five Basic Steps
- There are generally five separate steps in conducting a meta-analysis:
- Define your hypothesis – From readhing literature, define, set boundaries, etc.
- Locate the Literature – The central
- Identify and input data – Gather empiricial findings from primary studies (e.g., p-value, effect size, etc) and input into statistical database.
- Cacluate Effect Sizes – Calculate the overall effect -- effect size, significance level or confidence intervals associated with the effect size, and variaiblity (homogeneity) of the effect.
- Analyze Variables – Code your study variables, input the data into the database, and analyze the results. There are two types of study variables: (a) objective variables -- such as type of IV or DV, ..., (b) subjective variables -- inferential judgements made by two or more judges....
How do I conduct a meta-analysis?
First, choose which statistical approach suits your needs
- There are generally three different statistical approaches to conduct a meta-analysis so first you need to choose which approach best fits your needs. For an excellent detailed comparison of these three approaches, see (Johnson, Mullen, & Salas, 1995) (Comparison of Three Major Meta-Analytic Approaches. Journal of Applied Psychology, 80, 94-106). Some basic information from that article is posted below to get you started:
- Hedges & Olkin Approach – see (Hedges, 1981); (Hedges, 1982); (Hedges & Olkin, 1985)
- Rosenthal & Rubin Approach – see (Rosenthal, 1991); (Rosenthal & Rubin, 1978); (Rosenthal & Rubin, 1988)
- Hunter, Schmidt, & Jackson - see (Hunter, Schmidt, & Jackson, 1982); (Hunter & Schmidt, 1990)
Second, choose which effect size to calculate?
Third, choose your statistical software
DSTAT calculates all of this for you Can also use SPSS and macros from “Practical Meta-Analysis” Calculate Categorical variables – DSTAT using weighted ANOVA Calculate Continuous variables – SPSS using weighted Regression
If you want more detailed information about...
...the Hedges & Olkin approach...
- See (Lipsey & Wilson, 2001) (Practical Meta-Anlaysis) - which is relatively new book that provides a concise summary of all stages of the meta-analyses process, including providing ...
- See (Cooper & Hedges, 1994) (Handbook of Research Synthesis) - which is great in-depth articuluation of every step involved in designing, analyzing, and writing-up a meta-analysis.
- See (Hedges & Olkin, 1985) (Statistical Methods for Meta-Analysis) - which is the original source of information about the Hedges & Olkin approach.
...the Rosenthal & Rubin approach...
- See (Rosenthal, 1991) (Meta-analytic Procedures for Social Research) - which is the definitive source of information on the Rosenthal & Rubin approach.
- See (Rosenthal & DiMatteo, 2001) (Meta-Analysis: Recent Developments in Quantitative Methods for Literature Reviews) - which is an updated summary of the Rosenthal approach.
- See (Rosenthal, 1995) (Writing Meta-Analytic Reviews - which is an excellent Psychological Bulletin article on how to write a meta-analysis.
...the Hunter, Schmidt, & Jackson approach...
- What is the good number of studies to have bare minimum for a meta-analysis? A meta-analysis with 10 studies have been published before but is not recommended.
- In a meta-analysis, have judge rate each variable across studies, one moderator at a time, instead of rating all variables in a single study before moving on to next study.
- With meta-analysis coding with a high number of studies to code, such as 75+, can have some coders rate the entire set, but can also have some coders (undergrads) code only a subset as long there is overlap, so that more than 1 judge is rating each study.
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