NAEP Assessment Weighting Procedures → Weighting Procedures for the 2009 Assessment → Computation of Full-Sample Weights for the 2009 Assessment → School and Student Weight Trimming Adjustments for the 2009 Assessment → Trimming of Student Weights for the 2009 Assessment

NAEP Technical DocumentationTrimming of Student Weights for the 2009 Assessment

Large student weights generally come from compounding nonresponse adjustments at the school and student levels with artificially low school selection probabilities, which can result from inaccurate enrollment data on the school frame used to define the school size measure. Even though measures are in place to limit the number and size of excessively large weights—such as the implementation of adjustment factor size constraints in both the school and student nonresponse procedures and the use of the school trimming procedure—large student weights can occur due to compounding effects of the various weighting components.

The student weight trimming procedure uses a multiple median rule to detect excessively large student weights. Any student weight within a given trimming group greater than a specified multiple of the median weight value of the given trimming group has its weight scaled back to that threshold. Student weight trimming was implemented separately by grade, school type (public or private), and subject. The multiples used were 3.5 for public school trimming groups and 4.5 for private school trimming groups. Trimming groups were defined by jurisdiction and Trial Urban District Assessment (TUDA) districts for the public school samples for reading, mathematics, and science at grades 4 and 8; by states for the state component and by the balance of the nation for the national component for the public school samples for reading, mathematics, and science at grade 12; by the nation for the public school samples for science HOT and science ICT at grades 4, 8, and 12; and by affiliation (Catholic and non-Catholic) for the private school samples for all subjects at grades 4, 8, and 12.

The procedure computes the median of the nonresponse-adjusted student weights in the trimming group g for a given grade and subject sample. Any student k with a weight more than M times the median received a trimming factor calculated as follows:

where

• M is the trimming multiple,

• MEDIAN_g is the median of nonresponse-adjusted student weights in trimming group g, and

• STUWGT_gk is the weight after student nonresponse adjustment for student k in trimming group g.

In the 2009 assessment, relatively few students had weights considered excessively large. Out of the approximately 1,145,000 students included in the combined 2009 assessment samples, about 280 students required student weight trimming.

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