The interaction between z and y is the key to testing for nonuniform DIF. Item i is regressed on the interaction (y i ¼ aiy + aiz+oiyz+3i), which specifies that the relationship between itemresponse and z depends on the level of y. To carry out DIF testing, models are compared using LR tests. An omnibus test evaluates DIF for one item at a time (called ‘‘omnibus’’ because uniform and nonuniform DIF are tested simultaneously). First, a full model is fitted in which all studied items are regressed on z and the interaction. Next, one constrained model is fitted per studied item. In these models, the studied item is regressed on neither z nor the interaction. A statistically significant difference between the full and constrained models suggests that the item potentially functions differently between groups. Statistically significant effects from the full model indicate whether the item shows uniform DIF (bi), nonuniform DIF (oi), or both.