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 Model, Reading National Assessment, by Scale and Grade: 2009

NAEP Technical DocumentationProportion of scale score variance accounted for by the population-structure model, reading national assessment, by scale and grade: 2009

Grade	Number of initial  group-defining contrasts2	Number of principal components2	Proportion1 of scale score variance by the population-structure model
			Literary	Information
1 (Total Variance – Residual Variance)/Total Variance, where Total Variance consists of both sampling and measurement error variance. 2 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), 2009 Reading National Assessment.
4	4,476	1,072	0.53	0.55
8	4,320	920	0.55	0.60
12	3,340	487	0.58	0.58

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