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 weights for such schools may be large relative to what they would have been if these schools 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 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 new 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. (Note that in 2006, a factor of four instead of three was used.) The calculation of the school-level trimming adjustment factor for a new school s is expressed in the following formula:
EXP_WTs is the ideal base weight the school would have received if it had been on the original sampling frame, and
SCH_BWTs is the actual school base weight the school received as a sampled new school.
Eight new schools had their weights trimmed: three at grade 4 and five at grade 8.
Private schools eligible for weight trimming were Private School Universe Survey (PSS) nonrespondents who were found subsequently to have either larger enrollments than assumed at the time of sampling, or an atypical probability of selection given their affiliation, the latter being unknown at the time of sampling. For private school s, the formula for computing the school-level weight trimming adjustment factor SCH_TRIMs is identical to that used for new schools. For private schools,
EXP_WTs 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_BWTs is the actual school base weight the school received as a sampled private school.
Nine private schools had their weights trimmed: four at grade 8 and five at grade 12.