# Meta-analysis

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Meta-analysis

## Where should I start?

- If you want to learn what is a meta-analysis...

- For the basics behind learning about meta-analyses
- For more in-depth

- If you want to start conducting a meta-analysis...

- For the basics

## What is a meta-analysis?

### Definition

### 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 (variablitiy), then we look for moderating variables that explain the variablitty? In other words, does the effect of X on Y differ with moderator variables?

### Five Basic Steps

Define your hypothesis Locate the Literature Identify and input data Cacluate Effect Sizes Analyze Variables

## How do I conduct a meta-analysis?

### First, choose what statistical approach suits your needs

### Second, choose which effect size to calculate?

### Third, choose your variables

### If you want to follow the Hedges & Olkin approach...

### If you want to follow the Rosenthal & Rubin approach...

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

- There are generally three different statistical approaches to conduct a meta-analysis so first you need to choose which approach best fits your needs.

Hedges & Olkin Approach – Hedges, 1981; Hedges, 1982; Hedges & Olkin, 1985

Rosenthal & Rubin Approach – Rosenthal, 1991; Rosenthal & Rubin, 1978; Rosenthal & Rubin, 1988

Hunter, Schmidt, & - Hunter, Jackson, 1982; Hunter & Schmidt, 1990

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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