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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
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
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Datacollectionandprocessing
Data collection
Before initiating the collection of data, countries must participate in a workshop to train their research coordinators RCs in the GYTS standard methodology and
procedures. his assures continuity across the regions, consistency in sample design and selection procedures, and questionnaire development ensuring the core remains
intact, and uniformity in field procedures for data collection. Once the training workshop is over, RCs return to their respective countries and
conduct the survey among students in selected schools. Each student completes a questionnaire with responses coded as filled in bubbles on answer sheets. After com-
pleting the data collection of GYTS, the RCs send the survey forms answer and header sheets and school and classroom level forms to CDC for data processing.
Data processing
Answer sheets received by CDC are scanned using optical scanning hardware. Scanned data files proceed through a data cleaning process that includes matching
record length to scanned format, reviewing non-responses out of range and missing and logic editing. Each data record is weight-adjusted for school, class and student
non-participation. Finally, all records are adjusted for grade and gender stratification. here is continuing interaction between CDC staff and the RC and regional office
while cleaning and editing the data file.
Data analysis
Once the data has been collected and processed, WHO and CDC, in collaboration with associate partners, conduct data analysis workshops to provide hands-on train-
ing to the country RCs for an in-depth analysis of their data sets. Analysis workshops include training in the use of EpiInfo free software that encompasses procedures for
analysing complex survey data and country report writing.
Data reporting
Once the data file is finalized, CDC produces 100+ Weighted Frequency Tables and 100+ Preferred Tables. CDC prepares a draft one-page fact sheet highlighting the
main GYTS findings. he final data file, tables and the fact sheet are sent to the cor- responding regional office via e-mail and hard copy.
1. Tabulated data: he raw data are used in calculations for tabulated data. As part of the data processing for GYTS, CDC prepares two types of tables, Weighted Fre-
quency and Preferred Tables: • he Weighted Frequency Tables are produced as separate tables for each question
in the country questionnaires. Tabulations are given for total, by gender and by grade.
• A set of Preferred Tables is produced by CDC, which translates each core question, based on historical classification and including cross comparisons, into variables
that are utilized as indicators to monitor tobacco activity within the country.
