NAEP Technical DocumentationTrimming of School Base Weights for the 2010 Assessment

Large school weights can occur for private schools based on an 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 larger enrollments than assumed at the time of sampling. (Note that, prior to sampling, the same nominal value was assigned to all schools with unknown enrollment.)

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 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 figure and a known affiliation). If the school base weight was more than three times the ideal weight, a trimming factor was calculated for the 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 is expressed in the following formula:

SCH underscore TRIM subscript s equals three times EXP underscore WT subscript s divided by SCH underscore BWT subsript s if open paren SCH underscore BWT subscript s divided by EXP underscore WT subscript s close paren is greater than three or equals one if otherwise

where

EXP_WT_s 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_BWT_s is the actual school base weight the school received as a sampled private school.

Thirteen private schools had their weights trimmed: six at grade 4, one at grade 8, and six at grade 12.

Last updated 07 September 2010 (SS)

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