Sample Size and the Precision of the Confidence Interval in Meta-analyses

Sample sizes affect the precision of the confidence interval for the common effect size in a meta-analysis, which includes a number of independent studies of varying sizes. This paper provides a simplified method to estimate the precision of the confidence interval for the common effect size by using the number of independent studies and the average sample size of the independent studies. The simplified method proves to be very accurate for the retrospective meta-analyses. Two examples are then given to show how an investigator can use the number of independent studies and the average sample size of the individual studies to assess the precision of the confidence interval in a prospective meta-analysis.