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