1. Sampling Presentation on workshop in Luxembourg 10.April 2008 Johan Heldal

2. Target Population The TP in a survey is the finite set of units we want to study and draw inferences on. In HES: Individual persons at specified ages within a specified geographical area: A country It will sometimes have to be restricted by practical considerations defined by the sampling frame.

3. Recommendations for TP Geographical coverage: The entire territory of a country. Time of reference (t): A fixed date (e.g. January 1.) of the year of the survey. Core pop: Permanent residents filling ≥ 25 and ≤ 65 in (t, t+1). Recommended extension: Down to age ≥ 18. No upper bound.

4. Critical topics Type of sampling frame Coverage of sampling frame Timeliness of the sampling frame: Not too old. Sample size Goal: Nationally representative data.

5. A good frame Should have good coverage of target units. Should contain few non- target units that cannot be identified. Should contain information relevant for stratification Should contain good contact information. Should be up-to-date

6. Sampling frames I Population Registers Have individual persons as units Contain information on age and gender Contain information on addresses. Of 32 countries listed, 23 say they have a PR 16/23 want to use it as a HES frame 14/16 update continuously, 2/16 yearly 11/16 cover entire TP 9/16 do both Census Lists 7 countries want to use CLs as frame 5/7 cover the entire TP 3/7 update continuously, 2 annually

7. Sampling frames II Electoral Rolls 3/26 countries want to use ER as frame 1/3 update continuously Centralized GP list of patients. May only cover persons having a doctor. May be subject to professional secrecy(?). Telephone directory Not complete enough Fixed phones are related to households, not persons. Cell phones not always registered on the carrier Better used as supplementary frame

8. Sampling frames III Postcode Address Files (England, Scotland) May have good coverage of households But do not contain information on eligible residents Have no information on age and gender Maps combined with small area population statistics (NHANES, USA) Other frames may be supplementary

9. Frame recommendations Preferred frame: Updated register or census lists with complete coverage, whenever available. Second choice: Updated postcode address files (England). Third choice: Maps in combination with census statistics (NHANES)

10. Sampling design recommendations I Primary Sampling Units (PSUs): Clinic areas Secondary Sampling Units (SSUs): Invitees to sampled clinics. PSU-strata: Subgroups of PSUs by geography and health related factors. Demographic strata: Target population by gender and 10 year age groups.

11. Sampling design recommendations II PPS-sampling: Sampling of PSUs in PSU-strata with Probabilities Proportional to population Size. Equal sample size for each gender by age stratum Equal probability within each gender by age stratum About equal sample size to each sampled PSU (clinic).

12. Sample size Two sample sizes: 1.Number of PSUs sampled (clinics): m 2.Number of persons to be invited: n Recommendations: –Minimum size for n: 3000 invitees. –Larger sample recommended, required if detail below national level desired. –m and n = n/m set by cost efficiency criteria

13. That’s all. Thank you for your attention.