How Many Studies Do You Need?

In this article, the authors outline methods for using fixed and random effects power analysis in the context of meta-analysis. Like statistical power analysis for primary studies, power analysis for meta-analysis can be done either prospectively or retrospectively and requires assumptions about parameters that are unknown. The authors provide some suggestions for thinking about these parameters, in particular for the random effects variance component. The authors also show how the typically uninformative retrospective power analysis can be made more informative. The authors then discuss the value of confidence intervals, show how they could be used in addition to or instead of retrospective power analysis, and also demonstrate that confidence intervals can convey information more effectively in some situations than power analyses alone. Finally, the authors take up the question “How many studies do you need to do a meta-analysis?” and show that, given the need for a conclusion, the answer is “two studies,” because all other synthesis techniques are less transparent and/or are less likely to be valid. For systematic reviewers who choose not to conduct a quantitative synthesis, the authors provide suggestions for both highlighting the current limitations in the research base and for displaying the characteristics and results of studies that were found to meet inclusion criteria.