Over the past 30 years, debates about the usefulness of the Phillips curve for explaining inflation have been ongoing. One of the reasons for the recurring debate about the existence of an inflation and unemployment tradeoff is that there have been several instances when large movements in the unemployment rate have elicited little response in the inflation rate. In principle, these episodes of horizontal movement are consistent with a Phillips curve relationship; they just require the curve to shift in the same direction as the unemployment rate. Econometric representations of the Phillips relationship usually incorporate factors that can cause the Phillips curve to shift over time. However, the literature has not yet provided a test of whether such factors are sufficient to explain the episodes of horizontal movement.

In this paper, the authors test the explanatory power of a piecewise linear specification of the Phillips relationship against a simple linear specification. The authors find that a piecewise linear specification of the Phillips curve provides a good characterization of inflation dynamics over the past 40 years and that the traditional shifters in the relationships are insufficient to characterize the episodes of horizontal movement. Apparently, the gap between the unemployment rate and the natural rate of unemployment must be outside of some threshold values before triggering a response in inflation. This suggests that monetary policy should aim to drive the unemployment rate lower until the lower limit of the inflation range is reached; however, such a strategy is complicated by the fact that the lower unemployment limit is estimated with uncertainty, leaving the policy implications open to debate.