Work out item difficulty index instantly with clear inputs, formula shown and shareable results.
Item difficulty is confusingly named: it is the proportion answering correctly, so a high p means an easy item. 78 of 120 gives p = 0.65. For a four-option multiple choice item the optimal difficulty is midway between chance and certainty, at 0.625, because that maximises the item's ability to discriminate.
Difficulty index
p = correct responses / total candidates
Optimal difficulty
p = (1 + 1/options) / 2
Because guessing raises the floor. With four options the effective floor is 0.25, so the midpoint of the usable range is 0.625.
Not always. A few easy items early in a test build confidence, but they contribute almost nothing to discriminating between candidates.