ampling procedure and sample size determination
To determine the sample size for the study population the following steps were used. Formula for single Population proportion was used. Assumptions for 5 % marginal error (d) and 95 % confidence interval (α =0.05) used. Estimated prevalence of textual exposure obtained from previous study was p = 0.65. Accordingly, total sample size was 770. For the selection of these respondents, multi-stage sampling technique was employed. There were ten preparatory schools in Hawassa city, two were public and eight were private schools. One public and three private schools were selected using simple random sampling technique. For the four schools, respondents were allocated using Population proportionate to size (PPS) technique. Here, student’s roster (list) was used as sampling frame. In each of these schools, students were assigned as Grade 11 and 12. From these grades, sections of students were selected by lottery method. Participants in each of selected section of students were selected by lottery method (using students’ attendance sheet). Figure 1 sampling procedure.