An Item Characteristic Curve (ICC) is a graphical representation of the relationship between item difficulty and the probability of a correct response for different IQ levels. The x-axis represents the IQ of the test taker. The y-axis represents the probability of a correct response to the question. The ICC is the most important graph in the Item Response Theory (IRT).
The ICC shows how difficult the question is and how well it discriminates (What is item discrimination?) between test-takers with different IQ levels. If the ICC is located on the left side of the figure, it indicates that the question is relatively easy, and someone with a low IQ already has a high chance of giving the correct answer.
If the curve is shifted to the right, it indicates that the item difficulty is high (high item difficulty), because a high IQ is needed to have a reasonable chance at giving the correct answer. The slope of the ICC indicates the (item discrimination) of the question, a steeper slope means a better discrimination and a flatter slope indicates a poor discrimination. The point where the slope is the steepest, is called the Point of Maximum Information, because this is the point where the question discriminates the most between different IQ levels.
Below are examples of ICC’s of an easy question and a difficult question.
In the first ICC, someone with an IQ of 90 has a 50% chance of giving the correct answer, and the average IQ is 100, so the question is answered correctly by more than half of the people.
For the question that belongs to this ICC, someone with an IQ of 118 has a 50% chance of giving the correct answer, and the average IQ is 100, so this question is answered correctly by less than half of the people. Someone with an IQ of 90 only has around 20% chance of getting the right answer to this question. The question that belongs to this ICC is much more difficult than the question that belongs to the other ICC.