Men aged 17–54 years and women aged 15–44 years were recruited with only one member of each cohabiting marital union selected at random to participate. At the second follow- up survey, extended entry criteria allowed men and women aged 15–54 years and both members of a marital union to participate. In the second survey (from site five onwards) and the third survey, migrants into the study areas were invited to participate. Behaviour data Participants were interviewed using a structured questionnaire on sociodemographic characteristics and sexual behaviour. Interviews were led by social science graduates who were familiar with the local language and culture but were not known locally. Interviewers were trained to adopt an informal and non-prejudicial approach and to build up a good rapport with respondents. Interviewers were matched for sex with respondents. An informal confidential voting interview (ICVI) method was used to collect sensitive behavioural data as a means of reducing social desirability bias.19 Participants placed slips containing their responses into a ‘‘voting’’ box, thus ensuring confidentiality. All data on age at first sex and age at first marriage were collected using the face-to-face interview method, even when the ICVI method was used to collect subsequent behavioural data in the same interview. Participants were asked if they had been married or were in a long-term or cohabiting relationship, which we classify together as being ‘‘married’’. In the first round of the survey, relation- ships of >6 months were defined as long-term, but in the subsequent two rounds this was changed to be relationships of >12 months. Ever married individuals were asked their age when they first entered such a relationship. In the section on sexual behaviour, participants were asked initially how old they were when they had sex for the first time. Subsequent questions on sexual behaviour were asked in order of increasing sensitivity. Analysis of data A number of methods have been developed to analyse data on age at first sex (which also apply to age at marriage). The data used are current status (the current age of the individual and whether they have ever had sex) and recall (retrospective reports of age at first sex).20 Survival analyses combine the two sources. These techniques can also be adapted to analyse data from successive surveys to assess trends in age at first sex and distinguish underlying trends from reporting errors.21 Survival analysis techniques were employed to analyse timing of first sex, with individuals represented as surviving until first Table 1 Definitions of reliable and unreliable reporting Category Consistent Inconsistent: can be corrected Inconsistent: can be estimated Unreliable: cannot be estimated or corrected 1 Number of age-at-event reports Report ( age at survey Report .age at survey 2 Both reports same AND both reports ( age at survey Reports differ by 1 year AND both reports ( age at survey 1 report ( age at survey Reports differ by .1 year AND both reports ( age at survey OR both reports . age at survey Supplement sex. In this scheme the failure event is starting sex and follow- up time is censored if individuals had not started sex by the time of their last interview. Censoring is independent (of the probability that sex occurs), since no information on sexual behaviour was obtained prior to recruitment. Kaplan-Meier failure curves were plotted to illustrate the cumulative onset of sexual activity. This procedure was also followed when analysing trends in age at marriage. Age at first sex and age at marriage were reported as years in integers, which correspond to age at last birthday at those events. A random proportion of a year was added to each age to capture the distribution of exact ages within a year. Reports of age at first sex and age at marriage were classified according to consistency to assess the quality of reporting. Four categories were defined to classify reports: (1) consistent; (2) inconsistent: can be corrected; (3) inconsistent: can be estimated; and (4) unreliable: cannot be estimated or corrected (table 1). Figure 1 Consistency of reporting age at first sex and age at marriage among men and women aged 15–54 years who reported in at least two surveys. 3 All reports same AND all reports ( age at survey 2 ages reported which differ by 1 year AND both reports ( age at survey >50% of reports same AND these reports ( age at survey ,50% of reports same OR >50% reports same AND reports . age at survey Imputed response Reported age Older of the 2 ages Most frequent report that is ( age at survey Mean of all reported ages Reliable Unreliable Sex Transm Infect 2009;85(Suppl I):i34–i40. doi:10.1136/sti.2008.033431 i35