Figure 2.

Funnel plots examining publication bias in studies examining association between heroin dependence and (A) the allelic distribution in serotonin transporter gene promoter polymorphism (5-httlpr), and (B) the allelic distribution in 17-bp variant number tandem repeats in intron 2 of 5-HTT gene (STin2). The plots describe the relationship between effect sizes (log odds ratio) of studies and their precisions (inverse of standard error). The empty dots and diamond represent included studies and pooled effect size, respectively. The black dots represent imputed studies meant to balance skewed distribution of included studies in the funnel plot. The black diamond represents pooled effect size when considering imputed studies.

Abbreviations: 5-httlpr, 5-hydroxytryptamine transporter-linked promotor region; STin2, serotonin transporter intron 2.