Two THEMES RUN through policy analysis: rules determining policy choice are functions of economic conditions; those rules may change over time. The themes reflect the views that actual policy behavior is purposeful, rather than arbitrary, and that good policy adapts to changes in the structure of the economy or to improvements in understanding how policy affects the economy.
A growing body of evidence finds that policy reaction functions vary substantially over different periods in the United States. In light of this evidence of regime shifts, which is reviewed in Section 1, it is surprising that there is little formal modeling of environments where ongoing regime change is stochastic, and the objects subject to change are parameters determining how the economy feeds back to policy choice.
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