I judge that this decline reflects not only a  2  BIS central bankers’ speeches  deceleration in the growth of capital stock and the total factor productivity (TFP) but also a constraint on economic growth imposed by a labor shortage, which has become increasingly evident in the construction, restaurant, retail, transportation, and tourism sectors, notably among SMEs.
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.