# Meta-analysis

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Meta-analysis

have you ever wanted to learn about meta-analysis or conduct a meta-analysis but didnt know where to start? This webpage is devoted to providing

## Where should I start?

If you want to learn __what is a meta-analysis__...

- For the
**basics**behind meta-analyses see below where we lay out:

- the definition of a meta-anlaysis,
- the three basic questions answered by a meta-analysis,
- the the five steps involved in a meta-analysis,

- For more
**in-depth**discussion and explanations, we recommend...

- start first with

- For the

If you want to learn __ how to start conducting a meta-analysis.__..

- For the
**basics**behind conducting a meta-analysis see below were we lay out:

- how to choose a statistical approach to conducting a meta-analysis,
- the [[
- the [[

- 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.

- For the

## 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 (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****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

- 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 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...

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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