Economic forecasts are not very good. In fact, they are completely terrible.

From Is Obama Toast? Handicapping the 2012 Election by Nate Silver

There is, however, another problem: economic forecasts are not very good. In fact, they are completely terrible. In November 1995, economists expected the economy to grow at 2.6 percent the next year; it actually zoomed upward by 4.4 percent. In November 2007, they expected it to grow at 2.5 percent, but it shrank by 3.3 percent, as the effects of the global financial crisis became manifest. Frighteningly enough, the margin of error on an economic forecast made a year in advance is about plus or minus 4 percent of G.D.P.