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Copyright © Allyn & Bacon (2007) Meta-Analysis A statistical way to combine several studies on a single topic A statistical way to combine several studies on a single topic Involves Involves –Computing an effect size for each study –Computing a weighted effect size for the series of studies Advances scientific understanding Advances scientific understanding

Copyright © Allyn & Bacon (2007) Meta-analysis Science builds on its cumulative knowledge Science builds on its cumulative knowledge But statistical procedures tend to discount information from a study if the findings fall short of statistical significance But statistical procedures tend to discount information from a study if the findings fall short of statistical significance Meta-analysis combines both significant and non-significant findings Meta-analysis combines both significant and non-significant findings

Type II Error Refers to failing to reject a null hypothesis that should have been rejected Refers to failing to reject a null hypothesis that should have been rejected –Some non-significant statistical tests are type II errors (due to insufficient power) –Normally, these non-significant studies are ignored (that is, never published) Meta-analysis deals with Type II errors by including these non-significant findings in the analysis Meta-analysis deals with Type II errors by including these non-significant findings in the analysis Copyright © Allyn & Bacon (2010)

Summary Meta-analysis statistically combines the results of studies of the same phenomena Meta-analysis statistically combines the results of studies of the same phenomena An alternative to the traditional literature review An alternative to the traditional literature review By objectively summing the effects from all studies, it goes beyond the question of statistical significance By objectively summing the effects from all studies, it goes beyond the question of statistical significance