Unpacking the recommended indicator for concurrent sexual partnerships.

Using sexual behaviour survey data, we examine the methodological choice for the time period underlying the UNAIDS Reference Group on Estimates, Modelling, and Projections recommended 'point prevalence' indicator for concurrency. The results confirm that 6 months before the interview is a good time point for calculating the recommended indicator, but this retrospective estimate is substantially lower than the estimate of concurrency based on the number of current partnerships. The discrepancy is only partially explained by disproportionate missing data in those with more sexual partners.