We present a simple, comprehensive method for assessing similarity between sex partners of a participant and demonstrate its application using data collected in 2015 as part of CDC's National HIV behavioral surveillance (NHBS) among persons who inject drugs (PWID). We found that the pairwise similarity between sex partners of a survey participant was high. The similarity between second-to-last and third-to-last partners in the past 3 months was significantly higher than that between last and second-to-last partner in partner type, frequency of sex acts, and the contextual characteristics of sex behavior at last sexual encounter. The proposed approach provides an innovative measure of the added value of multi-partner series. The empirical analysis suggests that querying additional sex partners contributes limited data to characterize a participant's sexual behaviors among NHBS PWID. Future studies should apply the proposed method to evaluate the added value of data on multiple sex partners among other populations.
Keywords: Behavioral surveillance; HIV; Pairwise similarity; Persons who inject drugs; Sex partners.