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Item Discrimination II

The point biserial correlation (PBC) measures the correlation between the correct answer (viewed as 1 = right and 0 = wrong) on an item and the total test score of all students. The PBC is sometimes preferred because it identifies items that correctly discriminate between high and low groups, as defined by the test as a whole instead of the upper and lower 27 percent of a group. ParScore reports this value as item discrimination in column 5 on the “Standard Item Analysis Report” attached.

Inspection of the attached report shows that the PBC can generate a substantially different measurement of item discrimination than the simple item discrimination difference described above. Often, however, the measures are in close agreement.

Generally, the higher the PBC the better the item discrimination, and thus, the effectiveness of the item. The following criteria may be used to evaluate test items.

by schrod69 — last modified Jun 13, 2012 08:13 AM