求高人：svyset county [pw=sampwgt], strata(state) fpc(ncounties) || school, fpc(nschools)是什么意思？？？以下是数据信息：
Consider data on American high school seniors, collected
following a multistage design
Sex, race, height, and weight were recorded
In the first stage of sampling, counties were independently
selected from each state
In the second stage, schools were selected within each chosen
county
Within each school, every attending senior took the survey

variable name type format label variable labelsex byte %9.0g sex 1=male, 2=female
race byte %9.0g race 1=white, 2=black, 3=other
height float %9.0g height (in.)
weight float %9.0g weight (lbs.)
sampwgt double %9.0g sampling weight
state byte %9.0g State ID (strata)
county byte %9.0g County ID (PSU)
school byte %9.0g School ID (SSU)
id int %9.0g Person ID
ncounties byte %9.0g Stage 1 FPC
nschools int %9.0g Stage 2 FPC
Sorted by: state county school