Few studies adjusted the analysis for socioeconomic status19,22,23,36 or for education and physical activity.16,42 In a separate survey performed on members of one source population, users of COX-2 inhibitors were more likely than nonusers to have lower educational attainment, obesity, and current smoking13; investigators estimated that these differences caused a 2% bias away from the null. This evidence concerns the gene PTGS2 and obesity due to melanocortin 4 receptor deficiency.