s These three categories certainly do not exhaust the set of possible identifying restrictions, but they're the ones most identifying restrictions in Keynesian macroeconometric models fall into. Other possible sorts of identifying restrictions include, for example, a priori knowledge about components of E and cross-equation restrictions across elements of the Aj's, B-s, and Cj's, neither of which is extensively used in Keynesian macroeconometrics.
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Lucas et al. (1979) studied this question.
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