The sample selected for LTT uses a complex multistage sampling design that involves sampling students from selected schools within selected geographic areas across the country. The sampling design has the following stages:
Each selected school that participates in the assessment and each student assessed represents a portion of the population of interest. Some smaller populations are oversampled to ensure sufficient representation. Therefore, sampling weights are needed to make valid inferences between the student samples and the respective populations from which they were drawn. Sampling weights adjust for disproportionate representation due to such oversampling.
The target population consists of 9-, 13-, and 17-year-old students. Eligibility for the age 9 and age 13 samples is based on calendar year. For example, for the 2008 long-term trend assessment, students in the age 9 sample were 9 years old on January 1, 2008, with birth months January 1998 through December 1998; and students in the age 13 sample were 13 years old on January 1, 2008, with birth months January 1994 through December 1994. Students eligible for the age 17 sample had to be 17 years old on October 1, 2008, with birth months October 1990 through September 1991.