becoming sexually active. The sample frame for the study consists of ninth-grade students in Colombian urban public secondary schools. Given the study’s interest in cross-classroom spillovers, it required enrolled schools to have at least two ninth-grade classes. Schools were also required to have at least one computer room with Internet access.<sup>10</sup> All participating administrators of the schools had to consent for their school to participate in the field experiment before knowing the results of the randomization. Schools agreed to facilitate data collection and coordination, to make a computer lab available for the prescribed time every week (if selected to implement the course), and to not substantially modify their sexual and reproductive health education for ninth-graders during the study. A short questionnaire for school principals at baseline revealed that sexual education in the sample was either nonexistent, a topic covered in biology class, or consisted of one or two visits per year by a health professional. Schools in the control group received a sports equipment package as compensation at the end of the study.

The sample consists of 69 public secondary schools recruited in 21 cities across Colombia.<sup>11</sup> From each school, two classrooms of ninth-graders were selected to participate in the study. If the school had more than two classrooms of ninth-graders, a pair was randomly selected by the researchers to partake in the study.

# Data Collection Strategy

Data collection consisted of three rounds of a self-administered in-school pencil and paper survey. These surveys were administered unannounced during class time, so as to minimize data collection costs and attrition. The pencil and paper survey strategy was chosen instead of a computer survey because treatment students would have more familiarity with computers at endline, hence unnecessarily biasing results. The baseline survey was fielded at the beginning of the academic year; the second one at the end of the semester (after taking the course), and the last survey was taken at the end of the academic year, that is, six months after the end of the course. The academic year bracketed the timeline for the final survey data collection since beyond that point there is substantial student attrition due to students switching schools, and more fun- damentally, groups get reshuffled going into the 10<sup>th</sup> grade. This meant that to interview the participants during the 10<sup>th</sup> grade the researchers would have had to either locate the students in their new groups for re-interviews or survey the whole cohort to locate the participants’ responses expost. The first was deemed unfeasible by school administrators while the latter option was beyond the budget for the study. Six months after the end of the course, students were offered a voucher for six condoms with a total market value of about US$5. The offer was made via an email for all students and additionally via an SMS message for those who provided a cell phone number (86 percent of the sample). Data on which students redeemed their vouchers at the local health clinic were obtained by matching the voucher number to the student to whom the voucher was sent. A timeline of the intervention and data collection strategy is presented in fig. 1.

On the other hand, sexual education should in principle be provided before sexual initiation to convey its full benefits. In the United States 15 percent of adolescents have sex before age 15 (Flanigan et al. 2006).

> 10 The study selected schools with a functioning computer lab connected to the Internet with at least one computer for every three students. On average schools had 38 computers with a ratio of approximately 1 participating student per computer.

- 11 The sample excludes rural public schools. In urban settings, it is common for schools in Colombia to have two shifts per day (morning and afternoon). A student is offered a place at a certain shift before the beginning of the school year, and once a school is selected, he/she cannot take classes in other shifts or switch shifts. Given the lack of interaction among children of different shifts, the study treats different shifts in the sample as different schools. Both shifts are used for 13 schools in the sample.