A model of test item dependency is presented and used to illustrate the effect that violations of local independence have on the behavior of item characteristic curves. The dependency model is flexible enough to simulate the interaction of a number of factors including item difficulty and item discrimination, varying degrees of item dependence, and item order or sequence effects. The model also provides for an ability-by-dependence interaction. Results suggest that the shift in an item's characteristic curve can be fairly dramatic, producing nonlogistic response probability curves. (Author/GDC)