The number of children in any particular grade in the United States is approximately 3.5 million, and the number of students selected for NAEP' national samples for any particular grade and subject is 7,000 or more. Therefore, NAEP' sample consists of approximately 0.2 percent of the student population for each grade and subject. Although only a very small percentage of the student population in each grade is assessed, NAEP estimates are accurate because they depend on the absolute number of students participating, not on the relative proportion of students. Thus, all or nearly all of the schools and students selected must participate in the assessment to ensure that the NAEP sample is truly representative of the nation' student population. To ensure accurate results, the relatively small samples of students selected for the NAEP assessments must be truly representative of the entire student population.
Every school has some known chance of being selected for the sample.
Within a selected school, all students within a participating grade have an equal chance of being selected. The probability of students and schools being selected into the sample varies based on factors such as grade, subject, public and nonpublic status, and so on. Uncertainty about the validity of the results can arise, however, if selected schools decide not to participate, if selected students are absent or refuse to participate, or if schools or students volunteer to join the NAEP sample. For this reason, NAEP encourages the participation of all those selected, but is not able to accept volunteers. To ensure an accurate representation, the samples are randomly selected from groups of schools that have been stratified by variables such as region of the country, extent of urbanization, percentage of minority enrollment, and median household income.