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 the 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, Age 9 Reading Long-Term Trend Assessment: 2008

NAEP Technical DocumentationProportion of variance of the group-defining contrasts accounted for by the principal components used in the population-structure model, age 9 reading long-term trend assessment: 2008

Type of contrast	Number of contrasts	Mean	Minimum	Maximum	Number of contrasts by proportion1 of variance explained
					0.00 - 0.70	0.71- 0.80	0.81- 0.90	0.91 - 0.95	0.96 - 1.00
1(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), 2008 Reading Long-Term Trend Assessment.
All contrasts	568	0.90	0.63	0.99	11	39	176	166	176
Student questionnaire	72	0.89	0.63	0.99	4	9	12	25	22
Major reporting variables	60	0.90	0.66	0.99	1	4	21	12	22
Interactions between reporting variables	396	0.90	0.63	0.99	6	24	128	116	122
SD/ELL questionnaire	40	0.90	0.76	0.99	0	2	15	13	10

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