Second, the theoretically consistent output gap that plays a central role in the NewKeynesian literature bears little resemblance to the three empirical measures presented in the paper. The theoretical output gap is defined as the deviation of output from its equilibrium level in the absence of nominal rigidities.
Given that the theoretically consistent output gap is model-dependant, this adds to the difficulty of defining a reliable and applicable objective function. All this indicates that, as no Central Bank should be committed to a pure instrument-rule, in the same way it should not commit to and make public a precise objective function.