Addressing challenges in reviewing e-cigarette evidence: proposing a new ‘Test for Dominance’ in systematic reviews

Background Current systematic review evidence on e-cigarettes as a smoking cessation tool is contested. Part of the reason for this is the difficulty of objectively assessing the limited and heterogeneous evidence base. The study aim was to develop a novel approach to testing the validity of systematic reviews and assessing the extent of the dominance of included primary studies. Methods Leave-one-out meta-analysis was used to measure dominance. The systematic review on e-cigarettes as a smoking cessation tool by Hanewinkel et al. was used as a proof of concept. Results The Hajek et al. study dominates the Hanewinkel et al. systematic review, exerting a disproportionate influence on the overall pooled effect. Conclusions This research has identified a gap in current approaches to validity testing in systematic reviews. The Test for Dominance could supplement existing testing, including tests for heterogeneity, sensitivity analysis, and subgroup analysis. When a single study dominates a systematic review, the findings of that review should be interpreted with caution and further randomised controlled trials undertaken to reduce such distortion.