# Meta-analysis

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## Revision as of 02:25, 18 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

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## 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 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.

## What is a meta-analysis?

### 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:
2. Locate the Literature
3. Identify and input data
4. Cacluate Effect Sizes
5. Analyze Variables

## How do I conduct a meta-analysis?

### First, choose what 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)

### If you want to follow the Hunter, Schmidt, & Jackson approach...

Just as an individual study collects data from many individuals (data points) that is statistically summarized to answer the question of interest, a meta-anlay

• 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, don’t have raters code conditions for which no effect sizes can be calculated.
• 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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