Sampling

Presentation on workshop in Luxembourg10.April 2008Johan 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 countryIt will sometimes have to be restricted by practical considerations defined by the sampling frame.

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.

Critical topics

Type of sampling frameCoverage of sampling frameTimeliness of the sampling frame: Not too old.Sample sizeGoal: Nationally representative data.

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 stratificationShould contain good contact information.Should be up-to-date

Sampling frames IPopulation Registers

Have individual persons as unitsContain information on age and genderContain information on addresses.Of 32 countries listed, 23 say they have a PR16/23 want to use it as a HES frame14/16 update continuously, 2/16 yearly11/16 cover entire TP9/16 do both

Census Lists7 countries want to use CLs as frame5/7 cover the entire TP3/7 update continuously, 2 annually

Sampling frames II

Electoral Rolls3/26 countries want to use ER as frame1/3 update continuously

Centralized GP list of patients.May only cover persons having a doctor.May be subject to professional secrecy(?).

Telephone directoryNot complete enoughFixed phones are related to households, not persons.Cell phones not always registered on the carrierBetter used as supplementary frame

Sampling frames III

Postcode Address Files (England, Scotland)

May have good coverage of householdsBut do not contain information on eligible residentsHave no information on age and gender

Maps combined with small area population statistics (NHANES, USA)Other frames may be supplementary

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)

Sampling design recommendations I

Primary Sampling Units (PSUs): Clinic areasSecondary 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.

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 stratumEqual probability within each gender by age stratumAbout equal sample size to each sampled PSU (clinic).

Sample size

Two sample sizes:1. Number of PSUs sampled (clinics): m2. 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

That’s all.Thank you for your

attention.