Meta-analysis is a method for statistically combining research results. Prior to the creation of meta-analytic techniques, research was reviewed by reading all articles on a topic and then drawing an overall conclusion. For example, a researcher investigating the relationship between a police officer’s age and use of force might search for every study on the topic, count the studies showing significant results, and then conclude, “Because 15 studies showed a significant relationship and 16 did not, it does not seem that an officer’s age is related to his or her use of force.” However, due to a variety of issues, such as small sample sizes, such conclusions were often wrong.

The goal of a meta-analysis is to arrive at one number, called the mean effect size, ...