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Sampling

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

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Sampling

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  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: • Number of PSUs sampled (clinics): m • 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.

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