Fig. 4

Item information function surfaces. Each panel represents the information function surface of an item, with corresponding item number below the panel. X- and Y-axis characterize the level of two latent traits respectively. Z-axis refers to the item information (observation direction = 45°). An item is most useful when the item information peaks. Owing to the categorical nature of the responses, all item information function surfaces have multiple peaks. Generally, items provide most information among patients with moderate severity (ability between –1 and 2). When both latent traits are close to –3 or 3, the information is approaching to zero