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From |
"Angel Rodriguez Laso" <angel.rodriguez@salud.madrid.org> |

To |
<statalist@hsphsun2.harvard.edu> |

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 * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: Definition of strata and PSUs when svysetting***From:*"Stas Kolenikov" <skolenik@gmail.com>

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