The national

samples for students at ages 9, 13, and 17 were chosen using a multistage design that involved drawing students from the sampled public and private schools across the country. Within each age group, the results from the assessed students were combined to provide accurate estimates of the overall performance of students in the nation.

Because each school that participated in the assessment, and each student assessed, represents only a portion of the population of interest, disproportionate representation of subgroups in the selected sample may occur. Results must be weighted to account for any such disproportionate representation. This includes

oversampling of schools with high concentrations of students from certain racial/ethnic groups and the lower sampling rates of students who attend small schools.

The weighted national school participation rates for the 2012 long-term trend assessments are presented below. National student participation rates for 9-, 13-, and 17-year-old students were 95 percent, 93 percent, and 88 percent, respectively.

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 school students. At all three ages, the school participation rates met the standards for reporting results separately for public schools in 2012 but not for private schools. Catholic school participation rates, however, did meet the standards in 2012 for reporting results separately, so results for Catholic schools are included.