The target population for the 2012 NAEP long-term trend assessments consisted of 9-, 13-, and 17-year-old students enrolled in public and private schools nationwide. Eligibility for the age 9 and age 13 samples was based on the calendar year: students in the age 9 sample were 9 years old on January 1, 2012, with birth months January 2002 through December 2002, and students in the age 13 sample were 13 years old on January 1, 2012, with birth months January 1998 through December 1998. Students eligible for the age 17 sample had to be 17 years old on October 1, 2012, with birth months October 1994 through September 1995.
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.