I have a question about proper subsetting of survey data. This is the scenario: A health survey targeting the general U.S. population has a first stage of sampling that begins by defining primary sampling units (PSUs) as counties, or groups of counties for sparsely-populated areas. These PSUs are stratified by state (Washington, DC is lumped in with Maryland), and two are selected within each stratum, for a total of 100 sampled PSUs. Thereafter, subsequent stages of sampling are done in a hierarchical manner down to the household level and, ultimately, one individual within the household is selected to participate in the survey. For variance estimation purposes, however, the survey administration team only releases stratum and PSU codes.
If you were estimating the rates of a particular health condition (e.g., Type 2 diabetes) captured by the survey for a region of the country, which you defined as a grouping of 8 states, could you subset the survey data set for all cases in the region?
I'm confused about whether this would be a proper subset of the data if subsetting as described above. I believe that it is not because the number of people with a health condition could vary from one county to the next in the sample, so the domain sample size is a random variable itself. However, I'm not totally positive since it seems like a logical way to subset your data.