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A meta-analysis reviews 50 published studies examining a new cognitive behavioral therapy (CBT) intervention for depression. The average effect size is large (). However, researchers survey corresponding authors and discover approximately 30 additional unpublished studies with null results (no significant effect). When these 30 unpublished studies are included, the average effect size drops to . Which phenomenon best explains this dramatic difference?
A meta-analysis reviews 50 published studies examining a new cognitive behavioral therapy (CBT) intervention for depression. The average effect size is large (). However, researchers survey corresponding authors and discover approximately 30 additional unpublished studies with null results (no significant effect). When these 30 unpublished studies are included, the average effect size drops to . Which phenomenon best explains this dramatic difference?