Basic%20Item%20Analysis

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Basic Item Analysis

• IF and ID

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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

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Calculate the IF for these items
IF 15/16 .94
(Brown p. 65)
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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.

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Calculating ID
ID (Item 1) 1.00 0.8 0.20
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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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