The difficulty index of a test item equals the difference in the proportions of examinees in the upper and lower groups on total test score who get the item right.

Question

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The difficulty index of a test item is typically calculated as the proportion of examinees who get the item right, not the difference in proportions between upper and lower groups. The description given refers more closely to the concept of an item's discrimination index, which measures how well an item distinguishes between higher and lower ability examinees.

The Difficulty Index of a Test Item Equals the Difference