Supplement 1 Department of Infectious Disease Epidemiology, Imperial College London, UK; 2 Biomedical Research and Training Institute, Harare, Zimbabwe Correspondence to: Ms I Cremin, Department of Infectious Disease Epidemiology, Imperial College London, St Mary’s Campus, Norfolk Place, London W2 1PG, UK; ide. cremin05@imperial.ac.uk Accepted 19 December 2008 This paper is freely available online under the BMJ Journals unlocked scheme, see http:// sti.bmj.com/info/unlocked.dtl i34 Measuring trends in age at first sex and age at marriage in Manicaland, Zimbabwe I Cremin,1 P Mushati,2 T Hallett,1 Z Mupambireyi,2 C Nyamukapa,2 G P Garnett,1 S Gregson1,2 ABSTRACT Objective: To identify reporting biases and to determine the influence of inconsistent reporting on observed trends in the timing of age at first sex and age at marriage. Methods: Longitudinal data from three rounds of a population-based cohort in eastern Zimbabwe were analysed. Reports of age at first sex and age at marriage from 6837 individuals attending multiple rounds were classified according to consistency. Survival analysis was used to identify trends in the timing of first sex and marriage. Results: In this population, women initiate sex and enter marriage at younger ages than men but spend much less time between first sex and marriage. Among those surveyed between 1998 and 2005, median ages at first sex and first marriage were 18.5 years and 21.4 years for men and 18.2 years and 18.5 years, respectively, for women aged 15–54 years. High levels of reports of both age at first sex and age at marriage among those attending multiple surveys were found to be unreliable. Excluding reports identified as unreliable from these analyses did not alter the observed trends in either age at first sex or age at marriage. Tracing birth cohorts as they aged revealed reporting biases, particularly among the youngest cohorts. Comparisons by birth cohorts, which span a period of .40 years, indicate that median age at first sex has remained constant over time for women but has declined gradually for men. Conclusions: Although many reports of age at first sex and age at marriage were found to be unreliable, inclusion of such reports did not result in artificial generation or suppression of trends. In the era of the HIV pandemic, the focus of analyses regarding timing of first sex and marriage has broadened from investigating their role as proximate determinants of fertility to encompass their associations with risk of acquiring HIV.1–3 Identifying trends in the timing of age at first sex and marriage is essential for evaluating the impact of the investment made in HIV prevention inter- ventions to promote abstinence (by delaying first sex), and to assess the contribution of these changes to any downturn in the epidemic.4–7 Delays in reported age at sexual debut have coincided with declines in the prevalence of HIV in Uganda and Zimbabwe.8–12 Mathematical mod- elling, however, has indicated that these changes are unlikely to have contributed substantially to reductions in lifetime risk of infection or HIV incidence.13 The relationship between age at marriage and risk of infection has received attention. Recently, Bongaarts argued that late marriage can facilitate the spread of HIV via a long period of premarital sexual activity.14 However, it is also possible for early marriage to pose an increased risk of infection if marriage leads to regular unprotected sex in serodiscordant partnerships.15 In general, the age at which first sex occurs lies within the later teenage years.16 17 Thus, changes in the median age at first sex of just a year or two are considered epidemiologically significant. However, even modest degrees of social desirability bias, recall bias, digit preference or random misreporting could potentially generate spurious trends. For example, social desirability would be expected to bias reports of age at first sex upwards. Slaymaker and Buckner found that declines in reporting of sex before age 15 in Zambia were partially due to a change in reporting bias, when comparing two survey methodologies and accounting for the sociodemographic composition of the survey sam- ples.18 The Manicaland cohort study in Zimbabwe, which asked individuals about the timing of first sex and marriage in three survey rounds spanning a period of 8 years, presents an opportunity to monitor trends in age at first sex and marriage as well as to gauge the consistency of reporting which may inform on the accuracy and reliability of these data. The objectives of this paper are to (1) quantify the level of inconsistent reporting and its influence on observed trends; (2) identify reporting biases; (3) describe the overall trends in age at first sex and age at marriage; and (4) describe the length of time spent single (between first sex and marriage) in this population. METHODS Study design The Manicaland HIV/STD Prevention Project is a population-based open cohort study in the Manicaland province of eastern Zimbabwe. The study design and procedures have been outlined in detail previously.12 Briefly, participants were recruited from 12 predominantly rural study sites which consisted of 4 subsistence farming areas, 2 small towns, 2 roadside trading areas and 4 forestry, coffee and tea estates. Local residents were enumerated initially in a household census. Eligible individuals (those present in a local house- hold for at least four nights in the past month and at the same time one year ago) were then invited to participate in the study. This baseline survey took place in a phased manner from July 1998 to February 2000. This survey was repeated as a first follow-up from 2001 to 2003 and second follow-up from 2003 to 2005. Sex Transm Infect 2009;85(Suppl I):i34–i40. doi:10.1136/sti.2008.033431