Problems may arise in measuring drug injection frequency and risk behavior and in using them as independent and dependent variables, respectively. It can be clearly seen that risk behavior in drug injection is highly correlated with injection frequency. That is, as the risk behaviors increase, the injection frequency rises because the risk behavior and the nonrisk behavior consist of the total frequency of drug injections. Such an overlap in measurement may give rise to the high correlation between these two variables, and it may not be appropriate to use one of them as the predictor of the other; however, the article told us, "Injection frequency was not correlated with depression severity (r=0.08, N=109, p=0.40)" (p. 1660). Although the authors discussed the possible mechanisms by which depression severity is significantly associated with risk behavior but not with injection frequency, it will require more effort to examine the association between depression severity and the frequency of drug injection.