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HomeTDWNAEP Analysis and ScalingEstimation of Population and Student Group DistributionsExamining the Population-Structure Models Used in NAEPProportion of Scale Score Variance Accounted for by the Population-Structure ModelsProportion of Scale Score Variance Accounted for by the Population-Structure Models, Reading Long-Term Trend Assessment, by Age: 2004

NAEP Technical DocumentationProportion of scale score variance accounted for by the population-structure models, reading long-term trend assessment, by age: 2004

Age	Number of initial conditioning contrasts1	Number of principal components1	Proportion2  of scale score variance
9	1,504	126	0.37
13	1,761	158	0.39
17	1,768	154	0.41
1 Excluding the constant term. 2 (Total Variance – Residual Variance)/Total Variance, where Total Variance consists of both sampling and measurement error variance. SOURCE: U.S. Department of Education, Institute of Education Sciences, National Center for Education Statistics, National Assessment of Educational Progress (NAEP), 2004 Reading Long-Term Trend Assessment.

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