I fitted a mixed effects meta-regression model with one dummy-coded predictor (USA vs. Europe). The output tells me that the predictor is significant. I then wanted to get the average estimated effect size (and CI) for the US and for Europe. I used the predict() function. However when I look at the confidence intervals, they slightly overlap. Why is the p-value of the predictor in the meta-regression model significant, but the confidence intervals of the average effect size in each group overlap?
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Also related: https://stats.stackexchange.com/questions/497338/why-is-mean-%C2%B1-2sem-95-confidence-interval-overlapping-but-the-p-value-is-0 – COOLSerdash Nov 03 '22 at 08:22