In a meta-analysis displayed as a funnel plot, smaller studies are scattered asymmetrically away from the pooled estimate while larger trials cluster symmetrically near it. The most likely explanation for this pattern is:

• A High heterogeneity requiring a random effects model
• B Overadjustment for confounders in the primary trials
• C Reverse causation in the included cohorts
• D Publication bias favoring positive small studies ✓

Explanation

Under no publication bias, study estimates scatter symmetrically around the pooled value regardless of size. Asymmetry, with small studies showing exaggerated effects, classically reflects failure to publish small negative trials, since large trials get published almost regardless of result. Heterogeneity affects spread between studies, not funnel asymmetry, and reverse causation is a validity issue within individual observational studies.

Reference: Systematic Reviews in Health Care: Meta-analysis in Context (Egger, Davey Smith, Altman), 2nd ed.

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Written and medically reviewed by the StethoPrep medical team.