Metaanalysis
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► Have you ever wanted to learn about metaanalysis or conduct a metaanalysis 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 metaanalysis.
► With the thousands of metaanalyses conducted in all areas of psychology over the past few decades, there has been an everincreasing number of articles, books, and software programs devoted to how to conduct metaanalyses. Below, experts on metaanalysis provide their suggesstions on which which of the many sources of information are the most useful and why  so that the user has an easytouse starting place for learning everything about metaanalyses.
Contents 
Where should I start?
If you want to learn what is a metaanalysis...

If you want to learn how to start conducting a meta...

What is a metaanalysis?
Definition
A metaanalysis 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 metaanalysis 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 metaanalysis combines...
Three Basic Questions
 A metaanalysis answers three general questions:
 Central tendency – The central purpose of a metaanalysis 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 metaanalysis. 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 metaanalysis:
 Define your Hypothesis – First you must have a welldefined statement of the relationship between the variables under investigation so that you can carefully define the inclusion and exclusion criteria when locating potential studies. (see Chapter 2 of (Lipsey & Wilson, 2001) (Practical MetaAnlaysis) for a thorough examination of this step)
 Locate the Studies – A metaanalysis is only informative if it adequately summarizes the existing literature, so a thorough literature search is critical to retrieve every relevant study, such as database searches, ancestry approach, descendancy approach, hand searching, and the invisible college (e.g., network of researchers who know about unpublished studies, conference proceedings, etc)(see (Johnson & Eagly, 2000) (Handbook of Research Methods in Social and Personality Psychology) which details five general ways to retrieve relevant articles)
 Input data – Gather empiricial findings from primary studies (e.g., pvalue, effect size, etc) and input into statistical database. Not every study provides sufficient statistical information to calculate the effect size statistic.
 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 – If heterogeneity (variability) exists, you may want to analyze moderating variables by coding each variable in the database and analyzing either mean differences (e.g., categorical variables) or bivariate prediction (e.g., continuous variables) to see if the variable accounts for the variability in the effect size.
Two types of Variables
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....
such as publication type (e.g., articles, books, dissertation, technical reports, unpublished, etc) design features (e.g., experimental, correlational, random assignment control group
How do I conduct a metaanalysis?
First, choose which statistical approach suits your needs
 There are generally three different statistical approaches to conduct a metaanalysis 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 MetaAnalytic Approaches. Journal of Applied Psychology, 80, 94106). 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?
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Third, choose your statistical software
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DSTAT calculates all of this for you Can also use SPSS and macros from “Practical MetaAnalysis” 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 MetaAnlaysis)  which is relatively new book that provides a concise summary of all stages of the metaanalyses process, including providing ...
 See (Cooper & Hedges, 1994) (Handbook of Research Synthesis)  which is great indepth articuluation of every step involved in designing, analyzing, and writingup a metaanalysis.
 See (Hedges & Olkin, 1985) (Statistical Methods for MetaAnalysis)  which is the original source of information about the Hedges & Olkin approach.
...the Rosenthal & Rubin approach...
 See (Rosenthal, 1991) (Metaanalytic Procedures for Social Research)  which is the definitive source of information on the Rosenthal & Rubin approach.
 See (Rosenthal & DiMatteo, 2001) (MetaAnalysis: Recent Developments in Quantitative Methods for Literature Reviews)  which is an updated summary of the Rosenthal approach.
 See (Rosenthal, 1995) (Writing MetaAnalytic Reviews  which is an excellent Psychological Bulletin article on how to write a metaanalysis.
...the Hunter, Schmidt, & Jackson approach...
 What is the good number of studies to have bare minimum for a metaanalysis? A metaanalysis with 10 studies have been published before but is not recommended.
 In a metaanalysis, 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 metaanalysis 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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