On the Maddeningly Inexact Relationship Between Unemployment and Re-Election - Nate Silver

http://fivethirtyeight.blogs.nytimes.com/2011/06/02/on-the-maddeningly-inexact-relationship-between-unemployment-and-re-election/

The problem is that whatever signal there is gets filtered through an awful lot of noise. Consider:

The unemployment rate itself is subject to fairly significant measurement error.

Voters will interpret the unemployment rate in different ways, and assign the president varying amounts of credit or blame for it.

The unemployment rate is but one of a number of salient economic indicators.

Economic performance is but one of the ways that voters evaluate a president.

Voters’ evaluation of a president is important, but they also consider the the strength of a president’s opponents, including third-party alternatives in some elections.