In econometrics, model A is said to be nested in model B if it can be obtained from B by imposing a set of restrictions on the parameters of B. For example, model A1: y = α+βx+ε‎ is nested in model B: y = α+βx+γz+ε‎, but model A2: y = α+βx+δw+ε‎ is not. In this example, A1 can be obtained from B by imposing restriction γ = 0‎; A2 cannot be obtained from B as each contains a variable that is not in the other one.