NAEP Assessment Weighting Procedures → Weighting Procedures for the 2008 Assessment → Weighting Procedures for the 2008 LTT Assessment → Computation of Full-Sample Weights for the 2008 LTT Assessment → Weight Trimming Adjustments for the 2008 LTT Assessment → Trimming of School Base Weights for the 2008 LTT Assessment

NAEP Technical DocumentationTrimming of School Base Weights for the 2008 LTT 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 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 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:

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.

About 10 private schools had their weights trimmed for each age.

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