NAEP Technical DocumentationTrimming of Student Weights for the 2008 LTT 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. Student weight trimming was implemented separately by age, school type (public or private), and subject.

The procedure computes the median of the nonresponse-adjusted student weights in the trimming group g for a given age and subject sample. Any student k with a weight more than M times the median (where M = 3.5 for public schools and 4.5 for 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 2008 long-term trend (LTT) assessment, relatively few students had weights considered excessively large. Out of approximately 53,000 students assessed in the 2008 LTT assessment, about 200 students required student weight trimming.

Last updated 02 August 2010 (JL)

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