Controlling for rater effects when comparing survey items with incomplete Likert data

The rating scale model (Andrich, 1978) was applied to data from a survey that directed students to rate their satisfaction with college services on a five point Likert scale. Because students used different services, and students were directed to rate only the services they used, the items were differentially exposed to a person factor that we call "pleasability." Differential exposure to pleasability makes items' average rating a biased measure of their performance. In contrast, item parameter estimates in the rating scale model corrected for differential exposure to pleasability. Compared to items' average ratings, item parameter estimates in the rating scale model did a better job of predicting which item received the higher rating when any two items were rated by the same rater.