From Janet Yellen’s speech:
The economic outlook, of course, is highly uncertain and it is conceivable, for example, that inflation could remain appreciably below our 2 percent target despite the apparent anchoring of inflation expectations.
Economists’ understanding of the dynamics of inflation is far from perfect. Reflecting that limited understanding, the predictions of our models often err, sometimes significantly so. Accordingly, inflation may rise more slowly or rapidly than the Committee currently anticipates; should such a development occur, we would need to adjust the stance of policy in response.
Considerable uncertainties also surround the outlook for economic activity. For example, we cannot be certain about the pace at which the headwinds still restraining the domestic economy will continue to fade. Moreover, net exports have served as a significant drag on growth over the past year and recent global economic and financial developments highlight the risk that a slowdown in foreign growth might restrain U.S. economic activity somewhat further. The Committee is monitoring developments abroad, but we do not currently anticipate that the effects of these recent developments on the U.S. economy will prove to be large enough to have a significant effect on the path for policy. That said, in response to surprises affecting the outlook for economic activity, as with those affecting inflation, the FOMC would need to adjust the stance of policy so that our actions remain consistent with inflation returning to our 2 percent objective over the medium term in the context of maximum employment.
Given the highly uncertain nature of the outlook, one might ask: Why not hold off raising the federal funds rate until the economy has reached full employment and inflation is actually back at 2 percent? The difficulty with this strategy is that monetary policy affects real activity and inflation with a substantial lag. If the FOMC were to delay the start of the policy normalization process for too long, we would likely end up having to tighten policy relatively abruptly to keep the economy from significantly overshooting both of our goals. Such an abrupt tightening would risk disrupting financial markets and perhaps even inadvertently push the economy into recession. In addition, continuing to hold short-term interest rates near zero well after real activity has returned to normal and headwinds have faded could encourage excessive leverage and other forms of inappropriate risk-taking that might undermine financial stability. For these reasons, the more prudent strategy is to begin tightening in a timely fashion and at a gradual pace, adjusting policy as needed in light of incoming data.