Figure 1. Measuring partnership lengths, gap lengths, and overlap lengths.

We calculated the partnership length for each reported partnership as the time between the first sexual contact (F_partner) and the most recent sexual contact (MR_partner) with that partner (Figure 1A). For participants reporting two partners in the prior two months, we calculated the gap length between partners A and B as MR_A– F_B. Because times of first and most recent sex were reported as (or converted to) the number of days before the clinic visit, earlier events have larger values. Therefore, positive gap lengths correspond to situations of consecutive partnerships (Figure 1A). Figures 1B and 1C illustrate situations of concurrency in which the most recent contact with partner A (MR_A) occurred after the first contact with partner B (F_B). In these situations, F_B > MR_A and the gap length <0. In Figure 1B, one partnership is entirely contained within another, and the overlap length (diagonally hatched bar) is equal to the partnership length of the subsumed partnership. In Figure 1C, overlap is only partial; the overlap length is equal to F_B– MR_A. Figure 1D illustrates a situation of concurrency in which the most recent contact with partner A (MR_A) occurred on the same day as the first contact with partner B (F_B). In these situations, F_B = MR_A; therefore, the gap length =0 = the overlap length. For participants reporting a third partner in the prior two months (partner C), the gap lengths between – and overlap lengths across – partners B and C were calculated analogously.