Although non-response on the individual survey questions was generally very low, typically just one or two percent, for certain outcomes it could still result in slightly biased estimates of outcome measures if left unaddressed. The first set of these questions pertain to knowledge questions for example, "can you get pregnant if you have sexual intercourse only once?" where there is a single correct answer. For these questions, it is likely that youth who completed most of the survey section on knowledge, but skipped an individual question or two, did so because they did not know the correct answer. Thus, in order not to understate the proportion of youth who were unsure of a correct answer, the response on individually-skipped knowledge questions was categorized as "don't know/unsure." In contrast, youth who skipped an entire section are excluded from the analysis for that set of outcomes.
A more serious form of missing data pertains to conditional questions, meaning that they are answered by youth only if they provide a particular response on a prior question or questions. For example, in order to answer the question on the number of sexual partners, the respondent must first indicate on the survey that s/he has had sexual intercourse. Since youth who have not had sexual intercourse can correctly be assigned a value of zero partners, this conditional wording means that all missing values for the question will pertain to youth who have had sexual intercourse. In turn, unless there are no missing data, the reported mean value for the full sample will be incorrect in this case understating the mean number of sexual partners. To correct for this conditional item non-response, missing values were imputed following a commonly used "hotdeck procedure." This procedure assigns a value on the item that was missed based on the reported values of youth with characteristics similar to those of the item non-respondents. Through this method, the estimates for the program and control groups preserve the natural variability of the sample.