The Common Core of Data (CCD) file used to establish the sample frame corresponds to the 1999–2000 school year, whereas the assessment year is the 2001–2002 school year. In this two-year period, there were schools that closed, schools which changed structure (e.g., one school becoming two schools), and schools that came into existence. To achieve as close to full coverage as possible, the school sample was supplemented by a sample of new schools obtained from a sample of districts by sending them a list of the CCD schools and asking them to add in any new schools or old schools which were newly eligible for fourth or eighth grade. (To avoid undue burden on all school districts, a sample of districts was taken.)
The weights for new schools included in the sample were adjusted to reflect the district selection process probability, representing the unsampled districts.