Weight trimming is a weighting adjustment procedure that involves detecting and reducing extremely large weights. Extremely large weights generally mean large sampling weights that were not anticipated in the design of the sample. Unusually large weights are likely to produce large sampling variances of statistics of interest, especially when the large weights are associated with sample cases reflective of rare or atypical characteristics. To reduce the impact of these large weights on variances, weight reduction methods are typically employed. The motivation behind weight reduction methods is to reduce the mean square error of survey estimates. While the trimming of large weights reduces variances, it also introduces some bias. However, it is intended that the reduction in the variances more than compensates for the increase in the bias, thereby reducing the mean square error and thus improving the accuracy of survey estimates. NAEP employs weight trimming at both the school and student level.