In item-response theory, if a latent-structure model has an ability variable, then elementary information theory may be employed to provide a criterion for evaluation of the information the test provides concerning ability. This criterion may be considered even in cases in which the latent-structure model is not valid, although interpretation of the criterion is more complex in this case. It is also possible to consider reliability results for inferences about the ability variable. Use of sampling to develop required parameter estimates is straightforward. Applications of the criterion in zero-, one-, and two-parameter logistic (0PL, 1PL, and 2PL) examples are provided.