This means a +100 underdog is more likely to win than a +240 underdog (according to the book). In this system, the line number increases as the likelihood of winning decreases, and the line number decreases as the likelihood of winning increases. sportsbooks exclusively use what are called American odds, with positive figures (such as +100, +222, etc.) assigned to the underdog and negative figures (-120, -155, etc.) given to the favorite. Oddsmakers assign their probability through betting lines, which bettors see assigned to all moneylines, point spreads, totals and any other bet type. At its simplest, expected value in sports betting is a way to measure the probability gap between a bettor’s expectations - and the sportsbook’s.
