The schools and students participating in NAEP assessments are selected to be representative of all schools nationally. The results from the assessed students are combined to provide an accurate estimate of the overall performance of students in public, private, and other types of schools (i.e., Bureau of Indian Education schools and Department of Defense schools) in the nation.
Because each school that participated in the assessment, and each student assessed, represents a portion of the population of interest, the results are weighted to account for the disproportionate representation of the selected sample. This includes oversampling of schools with high concentrations of students from certain racial/ethnic groups and the lower sampling rates of students who attend very small schools. Read more technical information about weighting adjustments made at the school and student level.
School and student participation
To ensure unbiased samples, NAEP statistical standards require that participation rates for original school samples be 70 percent or higher to report national results separately for public and private schools. In instances where participation rates meet the 70 percent criteria but fall below 85 percent, a nonresponse bias analysis is conducted to determine if the responding school sample is not representative of the population, thereby introducing the potential for nonresponse bias.
The weighted national school participation rates for the 2011 writing assessment were 97 percent for grade 8 (100 percent for public schools and 71 percent for private schools), and 94 percent for grade 12 (96 percent for public schools and 67 percent for private schools). Weighted student participation rates were 94 percent at grade 8, and 87 percent at grade 12. Because the participation rate for private schools fell below 70 percent, results could not be reported for twelfth-graders attending private schools in 2011.
Nonresponse bias analyses were conducted for the private school samples at both grades 8 and 12. For grade 8, the results of the nonresponse bias analyses showed no significant bias for any school characteristic after substitution and nonresponse adjustments. However, at grade 12, some variables examined in the analyses still indicated potential bias after nonresponse adjustments. Specifically, the potential for bias still existed for race after nonresponse adjustments. Asian students and Native Hawaiian/Other Pacific Islander students were slightly underrepresented in the responding private schools sample.