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Basic Item Analysis

Basic Item Analysis. IF and ID. Item Facility (IF). Item facility tells us what percent of the test takers got the item correct. The calculation is simply the proportion of students who got the item correct to the total number of students. IF can range from 0.0 to 1.0.

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Basic Item Analysis

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  1. Basic Item Analysis IF and ID

  2. Item Facility (IF) • Item facility tells us what percent of the test takers got the item correct. • The calculation is simply the proportion of students who got the item correct to the total number of students. • IF can range from 0.0 to 1.0. • This number is sometimes called ‘difficulty index’ and denoted as ‘p’

  3. 15 correct 16 students total Calculate the IF for these items IF = 15/16 = .94 (Brown p. 65)

  4. Item Discrimination (ID) • Concept: If half of the students get an item correct (i.e , IF = .5), we expect the more proficient students to be in the half that got the item correct. • How can we define ‘proficient’? • The total score on the test. • Item Discrimination (ID) is an index of how well an item distinguishes between ‘more proficient’ and ‘less proficient’ students.

  5. Sorted by score Calculate ‘More proficient’ IF IF = 5/5 = 1.00 Calculate ‘Less proficient’ IF IF = 4/5 = 0.8 Calculating ID ID (Item 1) = 1.00 – 0.8 = 0.20

  6. Graphing Discrimination • For any given item, we expect the IF to be highest in the “more proficient” group, less in the “middle” group, and lowest in the “less proficient” group. • By graphing the IF values for each group, we get a visual representation of discrimination • These graphs are sometimes called Item Characteristic Curves (ICC)

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