To the Editor: We welcome the opportunity to respond to Mr. Shi’s comments concerning our article describing the association between depression severity and risky injection drug use, measured as the frequency of sharing needles.

One concern was our categorization of the intrinsically continuous response variable. As is commonly seen in studies of risk taking, the distribution of risky injection drug use was extremely skewed. The modal response (31.2%) was at the lower limit of zero and was not amenable to standard "normalizing" transformations, and the use of techniques like ordinal least-squares regression was untenable. Would we have observed the same association had we analyzed the continuous response? It depends on whether you use the product-moment (r=0.11, p=0.27) or rank-order (r_s=0.23, p<0.02) correlation coefficient. Many would dichotomize the outcome (any risk versus no risk) and use logistic regression. We did and the results were completely consistent with those reported; an increase of one standard deviation in depression severity increased the odds (adjusted for all covariates described in the article) of any use of dirty needles (risky injection drug use) by a factor of 1.92 (z=2.42, p<0.02). We constructed a four-category ordered response to better reflect the relative risk and estimated the association by using the ordered logit model. In preparing this response, we estimated the full model by using both rank-transformed and log-transformed continuous response variables. Although neither was approximately normal, the effect of depression severity was significant with both transformations, and we are convinced of the robustness of our finding. Without providing a conceptual rationale, Mr. Shi posits that HIV status and contact with drug treatment services may moderate rather than mediate the association between depression severity and high-risk behavior involving injected drugs. We found no evidence that the association between depression severity and risky injection drug use was conditioned by either treatment contact or HIV status (relevant p values exceeded 0.50). Given our relatively small group size, the statistical power to detect moderator effects was low, and these hypotheses may merit future inquiry.

We included total injection frequency as a covariate because we thought it a likely confound; its inclusion had virtually no substantive effect on the observed association between depression severity and risky injection drug use. Although it was not explicated in the published article, we examined the association between depression severity and total injection frequency by using a range of transformations, categorizations, and measures of association. There was no statistically reliable evidence that these variables were associated, and we discussed these issues.

To satisfy Journal word limits, articles must often present reconstructed summaries of deeper research processes. Most of Mr. Shi’s questions and concerns were addressed in the process of inquiry but were left on "the cutting room floor." Notwithstanding, the association of major depression and needle sharing has clear clinical implications and is worthy of continued study.