Large school weights can occur for schools selected from the NAEP new-school sampling frame and for private schools. New schools that are eligible for weight trimming are schools with a disproportionately large student enrollment in a particular grade from a school district that was selected with a low probability of selection. The school base weight for such schools may be large relative to what they would have been if they had been selected as part of the original sample.
To detect extremely large weights among new schools, a comparison was made between a new school's school base weight and its ideal weight (i.e., the weight that would have resulted had the school been selected from the original school sampling frame). If the school base weight was more than three times the ideal weight, a trimming factor was calculated for that school that scaled the base weight back to three times the ideal weight. The calculation of the school-level trimming factor for a new school s in jurisdiction j is expressed in the following formula:
EXP_WTjs is the ideal base weight the school would have received if it had been on the original sampling frame, and
SCH_BWTjs is the actual school base weight the school received as a sampled new school.
Three new schools had their weights trimmed: one at grade 4 and two at grade 8.
Private schools eligible for weight trimming were Private School Survey (PSS) nonrespondents who were found subsequently to have larger enrollments than assumed at the time of sampling. For private school s in jurisdiction j, the formula for computing the school-level weight trimming factor SCH_TRIMjs is identical to that used for new schools, except for the values of EXP_WTjs and SCH_BWTjs, which are defined as follows:
EXP_WTjs is the ideal base weight the school would have received if it had been on the original sampling frame with accurate enrollment and known affiliation, and
SCH_BWTjs is the actual school base weight the school received as a sampled private school.
Fourteen private schools had their weights trimmed: eight at grade 4, two at grade 8, and four at grade 12.