RT Journal A1 French JF T1 INtravenous cocaine and hiv infection JF JAMA JO JAMA YR 1989 FD September 15 VO 262 IS 11 SP 1471 OP 1471 DO 10.1001/jama.1989.03430110061022 UL http://dx.doi.org/10.1001/jama.1989.03430110061022 AB To the Editor. —  The article by Chaisson et al1 suggests that intravenous (IV) cocaine use is associated with human immunodeficiency virus infection. However, their data analysis contains methodological flaws and their conclusions make unjustified population generalizations.The multivariate model depicted in Table 1 of the article by Chaisson et al contains two interaction variables, "daily cocaine use, blacks and Hispanics," and "injection >3 times daily, blacks, before methadone treatment," yet includes only one of the four corresponding main effects (black race).The etiologic and mathematical interpretation of nonhierarchical models becomes complex.2,3 In the presence of a significant interaction, the meaning of the main terms is affected by how they are coded; hence, it is unwise to drop them from the model solely on the basis of tests of statistical significance. Hypotheses based on models that include significant interactions without their corresponding main effects are viewed by some