In a meta-analysis, the funnel plot shows marked asymmetry, with smaller trials clustered on the side favouring the intervention. What does this pattern most strongly suggest?

• A Random-effect model was inappropriate
• B Reverse causation
• C High clinical heterogeneity among trials
• D Publication bias or small-study effects ✓

Explanation

Under no bias, trial estimates scatter symmetrically around the pooled effect regardless of precision. Asymmetry driven by missing small negative trials indicates publication bias or related small-study effects such as selective reporting and poor methodology in small trials. Heterogeneity affects scatter width, not systematic asymmetry. Choice of fixed versus random effects concerns pooling weights, not plot shape. Reverse causation is a causal-inference problem in observational studies, not a meta-analytic plot finding.

Reference: Park's Textbook of Preventive and Social Medicine, 26th ed.

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