Seventy-five of 100 learners answer an item correctly. What is the item difficulty index p?

Correct answer: 0.75 The item difficulty index p is the proportion answering correctly: 75 ÷ 100 = 0.75.

• 0.25 0.25 is the proportion who answered incorrectly.
• 1.33 1.33 is 100 ÷ 75, the inverse of the proportion.
• 75 75 is the number (or percentage) correct, not the index.