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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 Variance Accounted for By Principal Components Used in NAEP Population-Structure Models → Proportion of Variance of the Group-Defining Contrasts Accounted for by the Principal Components Used in the Population-Structure Model, Grade 8 Writing Combined National and State Main Assessment: 2007

NAEP Technical DocumentationProportion of variance of the group-defining contrasts accounted for by the principal components used in the population-structure model, grade 8 writing combined national and state main assessment: 2007

Type of contrast	Number of contrasts	Mean	Minimum	Maximum	Number of contrasts by proportion1 of variance explained
					0.00 - 0.70	0.70 - 0.80	0.80 - 0.90	0.90 - 0.95	0.95 - 1.00
1 (Total Variance – Residual Variance)/Total Variance, where Total Variance consists of both sampling and measurement error variance. NOTE: NIES = National Indian Education Study; SD/ELL = Students with disabilities/English language learners. SOURCE: U.S. Department of Education, Institute of Education Sciences, National Center for Education Statistics, National Assessment of Educational Progress (NAEP), 2007 Writing Assessment.
All contrasts	1744	0.90	0.59	1.00	23	119	606	508	488
Student questionnaire	191	0.90	0.59	0.99	1	13	64	64	49
Major reporting variables	112	0.93	0.69	1.00	1	1	24	32	54
Interactions between reporting variables	622	0.91	0.60	0.99	11	31	150	202	228
Teacher questionnaire	257	0.90	0.67	0.99	2	13	112	55	75
School questionnaire	263	0.89	0.74	0.98	0	24	115	98	26
NIES questionnaire	260	0.87	0.64	0.99	8	34	131	48	39
SD/ELL questionnaire	39	0.92	0.75	0.99	0	3	10	9	17

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