With 2:08 left and the Colts with only one timeout, a successful 4th-and-2 conversion wins the game for all practical purposes. A conversion on 4th-and-2 would be successful 60 percent of the time. Historically, in a situation with 2:00 left and needing a TD to either win or tie, teams get the TD 53 percent of the time from that field position. The total win probability for the 4th-down conversion attempt would therefore be:
(0.60 * 1) + (0.40 * (1-0.53)) = 0.79 WP (WP stands for win probability)
A punt from the 28 typically nets 38 yards, starting the Colts at their 34. Teams historically get the TD 30 percent of the time in that situation. So the punt gives the Pats about a 0.70 WP.
Statistically, the better decision would be to go for it, and by a good amount.
--Brian Burke, NYT, on why Bill Belichick was right in going for it on fourth down. Part of a continuing series on this quasi-blog. See also here and here (search for "Bellman Equation").