Figure 1

Funnel plot. Figure 1 is a funnel plot of all studies included in the Cochrane systematic review: interventions for promoting smoking cessation during pregnancy [10]. The overall risk ratio for smoking cessation in each study is plotted on the x axis. The sample size of each study is plotted on the y axis. The plot illustrates possible publication bias with a gap caused by ‘missing’ studies upsetting the symmetry on the right hand side of the figure, indicating that small negative studies may not have reached the point of publication. Diamond shaped symbols indicate individual randomization whereas square symbols indicate that cluster randomization was used in the trial. The four black diamond shapes on the left side represent the four incentives trials included in the review. The dashed vertical line crosses the x axis at the weighted mean risk ratio of smoking reduction in all studies comparing intervention with control – 0.94 (95 % confidence interval 0.93 to 0.95).