Each person was assigned a probability of being infected with HIV in accordance to the prevalence rates recorded by the Botswana AIDS Impact Survey II (BIAS II), which averaged 17.1 per cent nationally, but varied across the population according to demographic and other factors.7 A random number between 0 and 1 was then assigned to each person. All people for whom the random number was less than or equal to the probability of being HIV-positive were deemed to be infected.