Item difficulty
Item difficulty is the proportion of students who answer a question correctly. Counter-intuitively, a higher value means an easier item.
An item difficulty of 0.9 means 90% got it right, so it is an easy question; 0.3 means only 30% did, so it is hard. A balanced paper spreads difficulty rather than bunching every question at one end.
Difficulty is not the same as quality. An item can be hard for the wrong reasons, such as confusing wording or an ambush vocabulary word, which is why it is read alongside how well the question separates stronger from weaker students.
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