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GLOBAL TOBACCO SURVEILLANCE SYSTEM
Global Youth Tobacco SurveyTraining Workshop
Introduction to the GYTS Sample Design &
Weights
Outline
Overview of GYTS survey design & sample selection– Target population– Design specifications & sample selection
Sampling design development– Sampling parameters worksheet– School enrollment list
Use of PC Sample software Sample weighting procedure
– Based on GYTS sample design and weights manual
No change in original GYTS sample design and selection
Cycle of Statistical Activity in GYTS Surveys
Population:
(Which rates to be
estimated; and for whom)
Sample Design:(Probability sampling; to
meet country’s information needs)
Selected Sample:
(Collect data from survey
respondents)
Analysis Findings:
(Produce design-based estimates of
rates)
Cycle of Statistical Activity in GYTS Surveys
Population:
(Which rates to be
estimated; and for whom)
Sample Design:(Probability sampling; to
meet country’s information needs)
Selected Sample:
(Collect data from survey
respondents)
Analysis Findings:
(Produce design-based estimates of
rates)
GYTS Target Population
School-based survey of students aged 13–15 years – By sampling grade range that includes this age group– Typically possible grade range 8-10
Nationally representative – National scope is the standard – To enhance consistency of findings among GYTS
countries by making the scope of findings the same– Geographic scope may be limited to subareas of the
country (e.g., certain regions, proximity to the capital city) & require clear justification
Includes students in public and private schools – Especially important if private/public school tobacco use
differs substantially– Inclusion of private schools depends on the availability
of lists of these schools – Require a clear justification if private schools are
excluded
Sample Sizes and Expected Precision
GYTS surveys are designed to produce nationally representative estimates, by gender, with a minimum student sample size of 1,500 from a minimum of 20 schools– A student sample size of at least 1,500 is recommended for
each domain (e.g., region/province or school type) when estimates are to be reported for these domains by gender
– Larger the sample sizes in terms of schools and students, higher the precision of estimates with lower margin of error
GYTS sample design should adjust for potential nonresponse and ineligibility at each stage– School participation rate – 80%– Student attendance rate – 80%
Cycle of Statistical Activity in GYTS Surveys
Population:
(Which rates to be
estimated; and for whom)
Sample Design:(Probability sampling; to
meet country’s information needs)
Selected Sample:
(Collect data from survey
respondents)
Analysis Findings:
(Produce design-based estimates of
rates)
GYTS Sample Design Specifications
Sampling is school based– Linkage to the target population via the schools
attended by members of the target populations Two stage sample design
1. Schools selected proportional to enrollment size Largest schools “self-representing” (i.e., selection
probability = 1) Stratification implicit from PPS systematic selection Explicit stratification as needed (e.g., by region,
urban/rural, etc.) Schools with fewer than 40 students may be excluded
2. Classrooms chosen randomly within selected schools All age-eligible students in selected classes eligible to
participate
Overview of GYTS Sample Design
V
V Select schools
Schools should be selected with probability proportional to size (PPS) Size measure is the school enrollment size Approximately 20 schools are selected for a target sample of 1,500 students
Select classes Classes should be selected systematically from each selected school Class selection is made at school by research coordinator Double draw of classes is considered based on GYTS coordination with other
surveys, as needed
Target Population Students in grades associated with 13-15 years of age
Include students in selected classes All the students in a selected class are eligible to participate in the survey
V
Sample Design Development
Sampling parameters worksheet– Implementing agency & contact– Scope of the survey– Type of schools– Type of sampling– Grades included & sample size– Grade & school level associated with
age group 13-15 years– Requirements of sample selection– Small school preference– GYTS coordination with other survey– Materials needed– Expected date for receiving school
enrollments & fieldwork period– Any other specifications
GYTS Sampling Parameters Worksheet
Date: ___/____/_____
Country:
Implementing Agency: __________________________________
Contact Name: _________________________________________
Mailing Address: _______________________________________
Telephone Number: _____________________________________
Email Address: _________________________________________
1. At what level is the GYTS to be implemented? (check all that apply): [ ] National level [ ] Regional level [ ] Both [ ] Other:____________________________________________ 2. Domain/strata for which estimates are required: a. Number:__________ b. List (e.g., region, type of school), please specify:________________ 3. What type of school is eligible for selection? [ ] Public [ ] Private [ ] Both [ ] Other, please specify:_________________________________ 4. Type of sampling: [ ] Sample [ ] Census 5. Grades and number of students to be sampled: a. Grades:__________ b. Number of students to be sampled (Target sample size):__________ 6. Which grades and school levels are associated with the age below in your country?
Age Grades School level
(e.g., primary, secondary, middle, high)
11 12 13 14 15 16 17
Sample Design Development
School enrollment list– School ID– School identification address– Province/state– Region– Type of school (private/public)– Grades taught– Level of school (e.g., primary, secondary
or high)– Enrollments by grade and sex– Total school enrollments in eligible grades– Any other specifications
Sample Selection Using PC Sample
School selection using PC Sample software– Within- school selection worksheets
Selection of schools using probability proportional to school enrollment size – School ID uniquely identify a school from the list– Uses PPS systematic sampling– Measure of size is the number of eligible students in each school– Certainty (self-representing) schools based on sampling interval– Re-compute the sampling interval for selection of the remaining
schools Selection Classes and Students
– Produces school-level forms and class-level forms for selection of classes
– Generate sequence of systematic random numbers for class selection within each selected school
All the students in the class completes the survey
Input files for PC Sample
School Enrollment List
Output files from PC Sample
School Selection List
Output files from PC Sample
School Level Sample Information Form
Class Selections Using School Level Sample Information Form
Output files from PC Sample
Class Level Sample Information Form
GYTS Sample Weighting Process
Sample weights– Base weight from student selection probability– Nonresponse adjustment (at school, class & student levels)– Calibrate the sample to the demographics of the target
population of students (by grade and sex) Weighted sample used to produce analysis findings
– Estimates of tobacco use prevalence & key tobacco control indicators
– Computing 95% CI’s using complex sample design features– Stratification and PSU will be identified for variance
estimation based on certainty and non-certainty schools in the sample
Sample design development: RC, MoH/MoE, and technical
experts
Send sample parameters worksheet & school enrollment
list templates to IA [RC]
Selection of schools & send school and class-level sample information forms to IA [RC]
Following instruction on school and class-level sample information forms for
implementation
Send completed school and class-level sample information forms for weighting and data
analysis
WHO/CDC work with IA [RC] to review and finalize sample
design
Sample Design Development Process
Process is confidential & at any stage the sample design, in part or full, should not
be disclosed or without the prior approval/permission from the IA [RC] &
CDC
Agenda for Sampling Breakout Session
Finalize sample design development & process
Review and finalize sampling parameters worksheet– Scope, target population & key
reporting sub-groups– Assess potential for coordinating
GYTS with another survey
Review school enrollment list
GLOBAL TOBACCO SURVEILLANCE SYSTEM
Global Youth Tobacco SurveyTraining Workshop
Introduction to the GYTS Sample Design &
Weights