NAEP Technical DocumentationTrimming of Student Weights for the 2014 Assessment

Large student weights generally come from compounding nonresponse adjustments at the school and student levels with artificially low first-stage 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 still occur.

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. Trimming groups were defined by region and Black/Hispanic strata for public schools and affiliation (Catholic/non-Catholic) for private schools. 

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 (where M = 3.5 for public and private schools) 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 2014 NAEP assessment, relatively few students had weights considered excessively large. Out of the approximately 51,700 students included in the combined 2014 assessment samples, approximately 200 students had their weights trimmed.

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