NAEP Analysis and Scaling → Initial Activities → Classical Item Analysis → Overall Descriptive Statistics for Items as Defined After Scaling, Grade 8 Science Combined National and State Assessments, By Block: 2011

NAEP Technical DocumentationOverall descriptive statistics for items as defined after scaling, grade 8 science combined national and state assessments, by block: 2011

Statistic	Item type	Block S3	Block S4	Block S5	Block S6	Block S7	Block S8	Block S9	Block S11	Block S12
† Not applicable. NOTE: The "weighted average item score" of polytomous items are standardized to have a scale of 0 to 1 so they are comparable to dichotomously scored items. The "weighted proportion of students attempting last item" is equal to 1 (percent not responding to last item /100). 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), 2011 Science Assessment.
Number of scaled items	Total	15	18	18	17	15	18	13	16	14
	Multiple choice	10	13	13	11	9	13	7	9	9
	Dichtomously scored constructed response	†	†	†	†	†	1	†	†	1
	Polytomously scored constructed response	5	5	5	6	6	4	6	7	4
Unweighted sample size	†	28,000	27,300	26,900	27,100	26,900	27,000	26,800	26,900	26,900
Weighted average item score	†	.54	.49	.47	.49	.42	.48	.42	.42	.45
Weighted average polyserial	†	.52	.52	.52	.51	.54	.54	.58	.54	.56
Weighted alpha reliability	†	.73	.74	.75	.75	.73	.73	.72	.73	.72
Weighted proportion of students attempting last item	†	.89	.82	.87	.90	.76	.89	.94	.75	.84

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