# st: Definition of strata and PSUs when svysetting

 From "Angel Rodriguez Laso" To Subject st: Definition of strata and PSUs when svysetting Date Tue, 25 Mar 2008 15:35:34 +0100

```Greetings to all members of the list,

I have the following questions on svysetting for an analysis of a complex
survey:

We have carried out a regional health population survey. We defined strata
initially as geographic areas in the region (n=7) and allocated to each of
them a sample proportional to their population. But because we wanted to
over-represent the elderly, we set that the number of people over 65 years
sampled in all areas had to reach a minimum number. We didn’t change the
sample size of people bellow 65 obtained through the proportional
allocation. Therefore the sampling fractions (and consequently the weights)
are different for each area by age group (bellow/over 65) category.

Then we selected census tracts in each geographic area with probabilities
proportional to their total population, and randomly sampled 10 individuals
in those selected, always keeping the proportion 7 bellow 65 years/3 over 65
years, which was the regional overall age distribution after the
oversampling explained above. My first question is if strata should be
defined as geographic regions alone or as geographic area by age groups
(bellow/ over 65 years) (n=14) when svysetting. The first possibility looks
more reasonable, because census tracts were selected within geographic
areas, not within geographic-age groups areas. If this is correct, then
probably the way to svyset would be declaring geographic areas as first
stage strata, census tracts as first stage PSUs and age groups as second
stage strata.

Alternatively, if the answer is that strata should be defined as region by
two age-groups categories, then the same census tract can belong to two
different strata (for example area A bellow 65/ area A over 65) depending on
the age of the individual considered. If I svyset: strata (region by age
group categories) and PSU= census tracts, STATA interprets that there are
twice the number of PSUs than real census tracts are. Is that correct?

Many thanks.

Ángel Rodríguez Laso
Institute of Public Health of the Region of Madrid

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