FRB/US is a large-scale quarterly econometric model of the U.S. economy, developed to replace the MPS model. Most behavioral equations are based on specifications of optimizing behavior containing explicit expectations of firms, households, and financial markets. Although expectations are explicit, the empirical fits of the structural descriptions of macroeconomic behavior are comparable to those of reduced-form time series models. In most instances, tests do not reject overidentifying restrictions of rational expectations or the hypothesis of serially independent residuals. As modeled, private sector expectations of policy constitute a major transmission channel of monetary policy.

This paper explores Knightian model uncertainty as a possible explanation of the considerable difference between estimated interest rate rules and optimal feedback descriptions of monetary policy. We focus on two types of uncertainty: (i) unstructured model uncertainty reflected in additive shock error processes that result from omitted-variable misspecifications, and (ii) structured model uncertainty, where one or more parameters are identified as the source of misspecification. For an estimated forward-looking model of the U.S. economy, we find that rules that are robust against uncertainty, the nature of which is unspecifiable, or against one-time parametric shifts, are more aggressive than the optimal linear quadratic rule. However, policies designed to protect the economy against the worst-case consequences of misspecified dynamics are less aggressive and turn out to be good approximations of the estimated rule. A possible drawback of such policies is that the losses incurred from protecting against worst-case scenarios are concentrated among the same business cycle frequencies that normally occupy the attention of policymakers.

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