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234 estimates. Given the same sample size of students, selecting a large number of schools, generally, yields more precise estimates than a sample of fewer schools. With the larg- er number of schools the average number of students selected per school is reduced, thus reducing the school “cluster” effect. Most countries implementing the GYTS select 25, 50 or 100 schools, depending on the statistical precision required, time frame for the fieldwork, and resources avail- able for conducting the survey. STAGE 2: Selection of classes sections and students Classes sections are randomly chosen from the selected schools. All students in the selected classes sections are eligible for participation in the survey. he number of students interviewed in most GYTS is between 1500 to 2000 students per sample site. Statistically a sample of 1500 students will yield representative estimates at a fair- ly precise level ± 5 for any population enrolment size. Once the sample size is set, the following process is followed. For a student sample of 1500 completed student interviews with an expected 80 student response rate, a sample of 1875 students is required. If 50 schools are selected at 80 participa- tion, then 40 will agree to participate. hus, 187540 = 47 students on average will be selected per school. his probably means one or two classes sections per school. Decreasing the number of schools to select will require selection of more classes per school. Increasing the sample size of students has the same effect of increasing the number of classes per school. Conducting฀the฀survey:฀practical฀considerations

1. Timing of the survey

he GYTS is now being conducted year-round in countries situated in both the northern and southern hemispheres. •฀ Determining the “best” time of year for conducting the GYTS will vary by coun- try, because the start and end dates for school differ. •฀ It is recommended that the GYTS be conducted during the middle of the morn- ing. he early part of the school day should be avoided because it could eliminate those students who arrive at school late. Lunchtime should be avoided. Later times of the day become poorer choices as students may leave school early.

2. Manpower resources

Because fieldwork should last less than two months, the number of available field staff will help determine how many schools can be selected

3. Other resources

he larger the sample size, the greater the cost in terms of printing of questionnaires and other supplies

4. NO replacement or substitution is allowed for schools that do not agree to participate