However, until a recent Australian study (E.C. Nelson, personal correspondence), most previous work used relatively low sample sizes with the number of cases ranging from 77 (Tan et al., 2003) to 486 (Xu et al., 2004). Previous analyses may thus have been statistically underpowered. The present study re-examines this issue as part of an ongoing study of opioid dependence in Bulgaria, using samples from ethnic Bulgarians and Romas, two previously unstudied populations.
