Large school weights can occur for schools selected through the new schools sampling procedure and for private schools. In 2006, no new public schools were selected. For this reason, only private schools were eligible for assessment of extreme school base weights. 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.
To detect extremely large weights among private schools, a comparison was made between a school's school base weight and its ideal weight (i.e., the weight it would have had had it been on the list frame portion of the PSS universe file at the time of NAEP school sampling, with an accurate enrollment figure and a known affiliation). If the school base weight was more than four times the ideal weight, a trimming factor was calculated for the school that scaled the base weight back to four times the ideal weight. (Note that in past years and in 2007 a factor of three was used instead of four. However, the factor of four is more appropriate for the sample design in 2006.) The calculation of the school-level trimming 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 list frame portion of the PSS universe file at the time of NAEP school sampling with an accurate enrollment and known affiliation, and
SCH_BWTs is the actual school base weight the school received as a sampled private school.
Twelve private schools had their weights trimmed. They all came from grade 12.