Public health researchers are addressing new research questions (e.g., effects of environmental tobacco smoke, Hurricane Katrina) for which the randomized controlled trial (RCT) may not be a feasible option. Drawing on the potential outcomes framework (Rubin Causal Model) and Campbellian perspectives, we consider alternative research designs that permit relatively strong causal inferences. In randomized encouragement designs, participants are randomly invited to participate in one of the treatment conditions, but are allowed to decide whether to receive treatment. In quantitative assignment designs, treatment is assigned on the basis of a quantitative measure (e.g., need, merit, risk). In observational studies, treatment assignment is unknown and presumed to be nonrandom. Major threats to the validity of each design and statistical strategies for mitigating those threats are presented.

Using epidemiological and social network research methods, this study examines gender differences in HIV risk and protective behaviors and social network characteristics among 193 young injection drug users (IDUs) and 127 referred members of their social networks. Respondents reported on their drug use, sexual behavior, and relationships within three types of social networks: hang out (i.e., friendship); drug use; and sexual networks. Most respondents were homeless and had experienced numerous life stressors. Females' social networks consisted more predominantly of drug injectors, and members more frequently appeared multiple networks. Females reported needle sharing more frequently than males, but also reported more protective behaviors such as needle exchange use and carrying clean syringes. Young female IDUs may compound their risk by having sex and injecting with higher risk partners. However, their propensity to practice protective behaviors may provide an opening for interventions to reduce their HIV risk and that of their social network members.

The objective of this paper is to profile the role of homelessness in drug and sexual risk in a population of young men who have sex with men (YMSM). Data are from a cross-sectional survey collected between 2001 in New York City (N = 569). With the goal of examining the import of homelessness in increased risk for the onset of drug and sexual risk, we compare and contrast three subgroups: (1) YMSM with no history of homelessness, (2) YMSM with a past history of homelessness but who were not homeless at the time of the interview, and (3) YMSM who were currently homeless. For each group, we describe the prevalence of a broad range of stressful life events (including foster care and runaway episodes, involvement in the criminal justice system, etc.), as well as selected mental health problems (including past suicide attempts, current depression, and selected help-seeking variables). Additionally, we examine the prevalence of selected drug and sexual risk, including exposure to a broad range of illegal substances, current use of illegal drugs, and prevalence of lifetime exposure to sex work. Finally, we use an event history analysis approach (time-event displays and paired t-test analysis) to examine the timing of negative life experiences and homelessness relative to the onset of drug and sexual risk. High levels of background negative life experiences and manifest mental health distress are seen in all three groups. Both a prior experience of homelessness and currently being homeless are both strongly associated with both higher levels of lifetime exposure to drug and sexual risk as well as higher levels of current drug and sexual risk. Onset of these risks occur earlier in both groups that have had an experience of housing instability (e.g., runaway, foster care, etc.) but are delayed or not present among YMSM with no history of housing instability. Few YMSM had used drug prior to becoming homeless. While causal inferences are subject to the limitations of a cross-sectional design, the findings pose an empirical challenge to the prevailing assumption that prior drug use is a dominant causal factor in YMSM becoming homeless. More broadly, the data illustrate the complexity of factors that must be accounted for, both in advancing our epidemiological understanding of the complexity of homelessness and its relationship to the onset of drug and sexual risk among high risk youth populations.