Chapter 24

Tertiary Studies:

Systematic Reviews &

Meta-Analyses

Overview

• A systematic review is the careful compilation and summary of all publications relevant to a particular research topic.

• A meta-analysis creates a summary statistic (a pooled statistic) for the results of systematically identified articles.

FIGURE 24- 1 Systematic Review Process

Search Strategy

• After identifying a well-defined study question, the next critical step in a systematic review or meta-analysis is to select appropriate databases, search terms, and search limiters.

• Once a system for identifying eligible articles is in place, abstract databases are systematically searched for articles that meet all the inclusion criteria.

FIGURE 24-2 Systematic Search

Strategy and Counts to Report

Data Extraction

Once all eligible articles are identified, the content of these articles is extracted into data extraction tables that list descriptive characteristics like:•The study location and study years•The study design•The study population and sample size•The key findings of interest•The strengths and limitations of the study

Data Extraction

• A data extraction table allows for easy compilation and comparison of observations relevant to the study question.

• Studies that find no statistically significant results for an item of interest are just as valuable as those that find a significant association.

• Publication bias occurs when articles with statistically significant results are more likely to be published that those with null results.

Meta-Analysis

• A meta-analysis pools the results of several studies identified during a systematic review to create one summary statistic.

• Only similar statistics from similar studies can be pooled.

• Before pooling the data, the researcher must show that the results of the studies are comparable.

Meta-Analysis

• A specialized computer software program can be used to estimate the value of the pooled statistic and its confidence interval.

• The contribution of each study to the pooled estimate is usually weighted based on the sample size of the included studies.

• The contributing studies and the summary measure are often displayed using a forest plot.

FIGURE 24-3 Example of a Forest Plot

Meta-Analysis

Threats to the validity of a meta-analysis: •Poor quality of included studies: The selection criteria used during the systematic review process can eliminate any studies of questionable validity. •Publication bias: The possibility of publication bias can be examined using a funnel plot.

FIGURE 24-4 Example of a Funnel Plot