NAEP Analysis and Scaling → Initial Activities → Classical Item Analysis → Block-Level Descriptive Statistics as Defined After Scaling, Grade 8 Science Combined National and State Main Assessment, by Public and Nonpublic School Sessions: 2009

NAEP Technical DocumentationBlock-level descriptive statistics as defined after scaling, grade 8 science combined national and state main assessment, by public and nonpublic school sessions: 2009

Statistic	Type of School1	Block S3	Block S4	Block S5	Block S6	Block S7	Block S8	Block S9	Block S10	Block S11	Block S12
† Not applicable. 1 Nonpublic-school sample includes only Catholic and private schools. 2 The "Weighted proportion of students attempting last item" is equal to 1 - (Percent not responding to last item /100). NOTE: Univariate scale. Item statistics in this table are described on the page Blocks of Items. SOURCE: U.S. Department of Education, Institute of Education Sciences, National Center for Education Statistics, National Assessment of Educational Progress (NAEP), 2009 Science Assessment.
Number of scaled items	†	15	18	18	17	15	18	13	17	16	15
Unweighted sample size	Public	29,500	29,400	29,300	29,300	29,200	29,300	29,100	29,100	29,100	29,100
	Nonpublic	600	600	600	600	600	600	600	600	600	600
Weighted average item score	Public	0.54	0.48	0.46	0.47	0.41	0.46	0.41	0.52	0.40	0.44
	Nonpublic	0.61	0.54	0.54	0.53	0.47	0.53	0.47	0.58	0.47	0.52
Weighted average polyserial	Public	0.52	0.53	0.51	0.52	0.54	0.54	0.58	0.57	0.54	0.57
	Nonpublic	0.48	0.48	0.51	0.47	0.50	0.51	0.57	0.54	0.50	0.53
Weighted alpha reliability	Public	0.73	0.74	0.74	0.75	0.73	0.74	0.71	0.78	0.73	0.75
	Nonpublic	0.67	0.66	0.71	0.67	0.67	0.69	0.69	0.73	0.70	0.68
Weighted proportion of students attempting last item2	Public	0.89	0.82	0.88	0.90	0.77	0.90	0.94	0.80	0.76	0.84
	Nonpublic	0.90	0.87	0.94	0.94	0.77	0.93	0.96	0.85	0.77	0.88

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