Each assessment cycle, a sample of students in designated grades within both public and private schools throughout the United States (and sometimes specified territories and possessions) is selected for assessment. In addition, in state assessment years, of which 2007 is an example, the samples of public schools and their students in each state are large enough to support state-level estimates. In all cases, the selection process utilizes a probability sample design in which every school and student has a chance to be selected, and standard errors can be calculated for the derived estimates.
The selection of a sample of public school students for state assessment involves a complex multistage sampling design with the following stages:
The Common Core of Data (CCD) file, a comprehensive list of operating public schools in each jurisdiction that is compiled each school year by the National Center for Education Statistics (NCES), is used as the sampling frame for the selection of sample schools. The CCD also contains information about grades served, enrollment, and location of each school. In addition to the CCD list, a set of specially sampled jurisdictions is contacted to determine if there are any newly formed public schools that were not included in the lists used as sampling frames. Considerable effort is expended to increase the survey coverage by locating public schools not included in the most recent CCD file.
As part of the selection process, public schools are combined into groups known as strata on the basis of various school characteristics related to achievement. These characteristics include the physical location of the school, extent of minority enrollment, state-based achievement scores, and median income of the area in which the school is located. Stratification of public schools occurs within each state. Grouping schools within strata by such selected characteristics provides a more ordered selection process with improved reliability of the assessment results.
On average, a sample of approximately 100 grade-eligible public schools is selected within each jurisdiction; within each school, about 60 students are selected for assessment. Both of these numbers may vary somewhat, depending on the number and enrollment size of the schools in a jurisdiction, and the scope of the assessment in the particular year. Students are sampled from a roster of individual names, not by whole classrooms. The total number of schools selected is a function of the number of grades to be assessed, the number of subjects to be assessed, and the number of states participating.
In years in which state-level samples are drawn for public schools, private schools are classified by type (e.g., Roman Catholic, Lutheran, etc.), and are grouped for sampling by geography (census division), degree of urbanization of location, and minority enrollment. About 700 private schools, on average, are included, with up to 60 students per school selected for assessment. These samples are not large enough to support state-level estimates for private schools. Thus, inferences for private schools are limited to the national level, even in years when public school assessments are state-specific.
A national sample of private schools in all grades is then drawn from a list compiled through the Private School Universe Survey (PSS), which is a mail survey of all U.S. private schools carried out biennially by the U.S. Census Bureau under contract to NCES. The PSS list is updated for new schools only for a sample of Roman Catholic dioceses.
In years when the NAEP samples are intended only to provide representation at the national level and not for each individual state, the public and private school selection process is somewhat different. Rather than selecting schools directly from lists of schools, the first stage of sampling involves selecting a sample of some 50 to 100 geographic primary sampling units (PSUs). Each PSU is composed of one or more counties. They vary in size considerably, and generally about 1,000 PSUs are created in total, from which a sample is selected. Within the set of selected PSUs, public and private school samples are selected using similar procedures to those described above for the direct sampling of schools from lists. The samples are clustered geographically, which results in a more efficient data collection process. The selection of PSUs is not necessary when the sample sizes are large in each state, as in state assessment years.