From the Business Journal : No Joke: Daily Show Viewers Follow Presidential Race

Polling conducted between July 15 and Sept. 19 among 19,013 adults showed that on a six-item political knowledge test people who did not watch any late-night comedy programs in the past week answered 2.62 items correctly, while viewers of Late Night with David Letterman on CBS answered 2.91, viewers of The Tonight Show with Jay Leno answered 2.95, and viewers of The Daily Show with Jon Stewart answered 3.59 items correctly. That meant there was a difference of 16 percentage points between Daily Show viewers and people who did not watch any late-night programming.

The study also includes stats on the subject of the jokes, and Stewart came out pretty balanced.