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NAEP Analysis and Scaling → Estimation of Population and Student Group Distributions → Examining the Population-Structure Models Used in NAEP → Proportion of Scale Score Variance Accounted for by the Population-Structure Models → Proportion of Scale Score Variance Accounted for by the Population-Structure Models, Writing Combined National and State Main Assessment, by Grade: 2002

NAEP Technical DocumentationProportion of scale score variance accounted for by the population-structure models, by grade, writing combined national and state main assessment: 2002

Grade	Number of initial group-defining contrasts1	Number of principal components1	Proportion of scale score variance for the writing scale
4	1,487	499	0.48
8	1,690	544	0.57
12	1,036	292	0.55
1 Excluding the constant term. SOURCE: U.S. Department of Education, Institute of Education Sciences, National Center for Education Statistics, National Assessment of Educational Progress (NAEP), 2002 Writing Assessment.

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