Fig. 3

Expected item score surfaces. Each panel represents the expected score 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 expected item score, i.e. the linear accumulation of probability of responding to each category of an item. Expected score reaches its peak when both latent traits were approximate to 3.0 (maximum). Latent trait 1 has greater impacts on responses to item 1–5, while latent trait 2 has greater impacts on responses to item 7–10. Item six has an approximately symmetric surface (the two latent traits have similar impacts on the item response)