BMC Public Health 2008, 8:136 http://www.biomedcentral.com/1471-2458/8/136 Health and National Population Commission. Data from the questionnaires were originally entered into the com- puter using CSPro 2.6 with in-built check commands. The data were subsequently imported into SPSS. A system of monitoring and quality management was implemented using multi-disciplinary Technical Committee drawn from a broad group of stakeholders. Independent reviews of the entire survey process were undertaken by technical advisors from USAID (through MEASURE Evaluation), DFID and WHO. The study received the approval of the Federal and the State Ministries of Health in Nigeria before implementa- tion. Furthermore, the consent or assent of individual par- ticipants was obtained after being fully informed of the study purpose and procedures. Specifically, the question- naire had a front page marked "confidentiality and con- sent", which was read to each respondent. Consents were freely obtained from each individual 15–17 years old but parental consent was not required. No name or other identifying information was maintained. Completed sur- veys were retrieved from interviewers daily by supervisors and centrally maintained until forwarded to the national coordination point immediately upon conclusion of the data collection. Measures The dependent variable was age of first sex, while inde- pendent variables grouped into three categories – demo- graphic, psychosocial, and community factors. Tables 1a and 1b show how each independent variable was defined and measured. Demographic factors consisted of age, gender, literacy in English language, education and economic status. "Wealth index" was constructed as proxy for economic status based on the method described by Filmer and Prit- tchett [11,12], which uses factor analysis to assign weights to specific household assets, type of dwelling structure and available utilities. Our sample was stratified into high, middle and low economic groups based on the fre- quency distribution of the overall scores. Psychosocial factors measured included religiosity, alco- hol use and personal attitude to premarital sex. Scales were constructed using exploratory factor analysis to assess other psychosocial factors: FP accessibility and atti- tudes; condoms perceived norms, efficacy and access; HIV prevention knowledge; gender attitude; and, sexuality- related communication (Table 2). Internal consistency and reliability of the scales was measured by Cronbach alpha, and the alpha model used is equivalent to the Kuder-Richardson 20 (KR20) coefficient for dichotomous data [13]. The community factors assessed were location (rural or urban), region (North or South) and media exposure. Data analysis Data analysis was undertaken using SPSS (version 12.0) and STATA (version 9.0). Sampling weights, based on the sampling fractions derived from final sample size and the population of eligible respondents for the states was applied in the analysis. Univariate analysis was carried out to determine the proportion of sexually experienced ado- lescents, age of sexual debut and its median. Chi-square was used to assess gender differences in sexual experience pattern. Based on the result, subsequent data analyses were gender-disaggregated. Our analyses aimed, on the one hand, at determining factors significantly associated with sexual debut. On the other hand, it also aimed to examine, among the sexually experienced adolescents, factors that may predict early (before the age of 16 years) and later sexual initiation. Age 16 years was chosen because it represented the median obtained for sexual ini- tiation. Chi-square (for categorical variables) and t-test (for continuous variables) were used to assess significance of bivariate associations. Multivariate analysis was carried out using Cox propor- tional hazards regression (survival analysis). In building the model, backward likelihood ratio (stepwise) approach was used for exploratory purposes, and the result obtained from the process alongside theoretical knowl- edge from the literature with regards to factors associated with sexual behaviour among young people were used in selecting variables for the regression models. Two models were presented: the first included only selected demo- graphic and community variables – factors that are likely to precede sexual initiation, while the second model included psychosocial factors in addition to the demo- graphic and community factors. For each of the two mod- els, hazard ratio (HR), 95% confidence interval (CI) and p-values were calculated separately for males and females. Literacy in English language and attendance frequency of attendance of religious services were not included in any of the models to avoid the problem of colinearity in view of their significant correlation with educational level and beliefs about the importance of religion respectively. With regards to the sexually experienced adolescents, given that very few variables were found to be significantly different between early and late sex initiators, survival analysis was not used to compare the two sub-groups. Statistical signif- icance was set at an alpha level of 0.05. Results Our final sample consisted of 1,195 males (57.7%) and 875 females (42.3%). A total of 1,661 (80.2%) reported never having had sexual intercourse (82.0% of males and 77.8% of females) (Table 3). For the sexually experienced Page 3 of 14 (page number not for citation purposes)