The trait level is depicted on the abscissa, with standard scores ranging from -3 to +3. An average amount of the trait in question would be indicated by a score of 0. Actually, for mathematical reasons, the scores in an IRF can range hypothetically from-∞ to+∞, but in actual practice, scores rarely escape the bounds of -3 to+3. The ordinate depicts the probability of a correct response on a scale from 0 to 1