Tobacco-related school curriculum ebooksclub.org Building Blocks for Tobacco Control A Handbook
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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. Conductingthesurvey:practicalconsiderations