GYTS is a school-based survey that collects data from students aged
13-15 years by using a standardized methodology for constructing the sample
frame, selecting participating schools and classes, and processing data. GYTS
uses a two-stage, cluster-sample design that produces representative samples
of students in grades associated with ages 13-15 years.2 In
the Philippines, this age range is covered by the second, third, and fourth
years of secondary school; the GYTS sampling frame included all secondary
schools containing these grade levels. At the first sampling stage, the probability
of selecting a school was proportional to the number of students enrolled
in the specified grades. At the second stage, classes within the selected
schools were randomly selected. All students attending school in the selected
classes on the day the survey was administered were eligible to participate.
In 2000, a total of 11,630 students completed the GYTS, and in 2003, a total
of 7,478 completed the survey. The school response rate was 90.0% in 2000
and 99.3% in 2003; the student response rates were 88.7% and 85.4%, respectively;
and the overall response rates (i.e., school rate multiplied by student rate)
were 79.7% and 84.8%, respectively.