NAEP Technical DocumentationTrimming of School Base Weights for the 2009 Assessment

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 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 is expressed in the following formula:

SCH underscore TRIM subscript s equals three times EXP underscore WT subscript s divided by SCH underscore BWT subscript 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 NAEP public school sampling frame, and
SCH_BWT_s is the actual school base weight the school received as a sampled school from the new school frame.

Twenty-two (22) new schools had their weights trimmed: six at grade 4, fourteen at grade 8, and two at grade 12.

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 factor SCH_TRIM_s is identical to that used for new schools. For private schools,

EXP_WT_s is the ideal base weight the school would have received if it had been on the NAEP private school sampling frame with accurate enrollment and known affiliation, and
SCH_BWT_s is the actual school base weight the school received as a sampled private school.

Four private schools had their weights trimmed: one each at grades 4 and 8 and two at grade 12.

Last updated 24 September 2014 (JS)

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