NAEP Technical DocumentationTrimming of Student Weights for the 2017 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 and the trimming groups are defined below.

State Public School Samples for Mathematics and Reading at Grades 4 and 8

• For these samples, the multiple used was 3.5, and the trimming groups were formed within each jurisdiction by Trial Urban District Assessment (TUDA) district vs. the balance of the state for states with TUDA districts.

National Public School Samples for Writing at Grades 4 and 8

• For these samples, the multiple used was 3.5, and the trimming groups were formed by dichotomy of low/high percentage of Black and Hispanic students (15 percent and below, above 15 percent).

National Private School Samples for Mathematics and Reading at Grades 4 and 8

• For these samples, the multiple used was 5.5, and the trimming groups were formed by affiliation (Catholic, Non-Catholic).

National Private School Samples for Writing at Grades 4 and 8

• For these samples, the multiple used was 4.5, and the trimming groups were formed by dichotomy of low/high percentage of Black and Hispanic students (15 percent and below, above 15 percent).

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 2017 assessment, relatively few students had weights considered excessively large. Out of the approximately 826,000 students included in the combined 2017 assessment samples for mathematics, reading, and writing, 149 students had their weights trimmed.

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