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Computation of Measures of Size
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The sampled schools for the eighth-grade private school national assessments in civics and U.S. history came from two frames: the primary private school sample frame constructed from the Private School Universe Survey (PSS) file and the supplemental new-school sampling frame. Schools were sampled from each school frame with probability proportional to size using systematic sampling. Prior to sampling, schools in each frame were sorted by the appropriate implicit stratification variables in a serpentine order within each explicit sampling stratum. (For details on explicit and implicit strata used for these samples see the stratification page.) A school's measure of size was a complex function of the school's estimated grade enrollment. Only one hit was allowed for each school.
Schools from the PSS-based frame were sampled at a rate that would yield a national sample of 1,600 assessed students (800 each from the Catholic and non-Catholic school strata across both subjects). Catholic schools from the new-school frames were sampled at the same rate as those from the PSS-based frame.