A note on the hierarchical model for responses and response times in tests of van der Linden (2007).

Findings suggest that in psychological tests not only the responses but also the times needed to give the responses are related to characteristics of the test taker. This observation has stimulated the development of latent trait models for the joint distribution of the responses and the response times. Such models are motivated by the hope to improve the estimation of the latent traits by additionally considering response time. In this article, the potential relevance of the response times for psychological assessment is explored for the model of van der Linden (Psychometrika 72:287-308, 2007) that seems to have become the standard approach to response time modeling in educational testing. It can be shown that the consideration of response times increases the information of the test. However, one also can prove that the contribution of the response times to the test information is bounded and has a simple limit.