# Meta-analysis

(Difference between revisions)
 Revision as of 04:45, 18 July 2006 (view source)Stenstro (Talk | contribs)← Older edit Revision as of 03:53, 21 July 2006 (view source)Stenstro (Talk | contribs) (→Five Basic Steps)Newer edit → Line 72: Line 72: ===Five Basic Steps=== ===Five Basic Steps=== :There are generally five separate steps in conducting a meta-analysis: :There are generally five separate steps in conducting a meta-analysis: - #Define your hypothesis – From readhing literature, define, set boundaries, etc. + #Define your hypothesis – First you must have a well-defined statement of the relationship between the variables under investigation so that you can carefully define the inclusion and exclusion criteria when locating potential studies. Chapter 2 of + + From readhing literature, define, set boundaries, etc. #Locate the Literature – The central #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. #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. #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.... #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?== ==How do I conduct a meta-analysis?==

## Revision as of 03:53, 21 July 2006

► 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... For the basics, see below where we lay out: the definition of a meta-anlaysis, the three basic questions answered by a meta, the the five steps involved in a meta-analysis, For more in-depth discussion and explanations, we recommend... start first with (Johnson & Eagly, 2000) which is a chapter from the "Handbook of Research Methods in Social and Personality Psychology" that provides an excellent detailed explanation of each stage in the meta-analysis process including the advantages of meta-analysis compared to traditional literature reviews, statistical forumlas for analyzing effect sizes from various indexes and study designs, and discussion of how to visually depict your data. then for even more in-depth descriptions see (Cooper & Hedges, 1994) (Handbook of Research Synthesis) which provides a separate chapter on every step involved in designing, analyzing, and writing-up a meta-analysis. If you want to learn how to start conducting a meta... For the basics, see below were we lay out: For more in-depth discussion and explanations, we recommend... start first with (Johnson, Mullen, & Salas, 1995) which provides a statistical comparision of the three major meta-analytic approaches using actual datasets, as well as the staistical forumulas for each approach and the methodological differences between each approach. based upon which meta-analytic approach you choose to use, see (Lipsey & Wilson, 2001) for the Hedges/Olkin approach, see (Rosenthal, 1991) for the information on the Rosenthal/Rubin approach, or see (Hunter, Schmidt, & Jackson, 1982) for the Hunter/Schmidt/Jackson approach.

## What is a meta-analysis?

### Definition

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:
1. 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?
2. 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.
3. 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:
1. Define your hypothesis – First you must have a well-defined statement of the relationship between the variables under investigation so that you can carefully define the inclusion and exclusion criteria when locating potential studies. Chapter 2 of

From readhing literature, define, set boundaries, etc.

1. Locate the Literature – The central
2. Identify and input data – Gather empiricial findings from primary studies (e.g., p-value, effect size, etc) and input into statistical database.
3. 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.
4. 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:
1. Hedges & Olkin Approach – see (Hedges, 1981); (Hedges, 1982); (Hedges & Olkin, 1985)
2. Rosenthal & Rubin Approach – see (Rosenthal, 1991); (Rosenthal & Rubin, 1978); (Rosenthal & Rubin, 1988)
3. Hunter, Schmidt, & Jackson - see (Hunter, Schmidt, & Jackson, 1982); (Hunter & Schmidt, 1990)

### Third, choose your statistical software

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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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