(yes/no); mean age (continuous variable); percentage of males (continuous variable); type of dependence (opioid/heroin); and majority ethnicity (Caucasian/Asian). The potential for publication bias in the meta-analysis was assessed statistically using Egger’s test31 and visually using Begg’s funnel plot.32 The funnel plot shows the standard error (SE) of the log OR, a measure of study precision, against the log OR. In the absence of bias the plot should be symmetric and funnel-shaped, with smaller less precise studies being more widely scattered around the overall estimate of the log OR than larger studies. An asymmetric plot is typically interpreted as providing evidence of publication bias. All calculations were performed using Stata version 10 (Stata Corp., College Station, TX, USA). A P-value < 0.05 was considered statistically significant.
