How the first half results change a team's outlook for the second half of a game

The lines for the 1st half of games are a simple enough prospect, and are often close to the full line on a game divided by two. There’s usually a slight penalty to the favorite (so a –7 line might be –4.5 for the favorite in the first half), but by and large you can predict the lines with some accuracy if you know the full spread. The sportsbooks can set the 1st half numbers well in advance of the game, and suffer no more than the typical exposure in the event of a bad line.

On the other hand, the second half lines that get set in the short time between halves are where the real adventure takes place, for the linemakers, the books, and the players who elect to jump in!

For these latter wagers, linemakers send over a hastily created second half line and the casinos book a flurry of bets in the few minutes before the game resumes. Clearly when posting a number with so little time to assess everything there is substantial room for error, and this is one reason why the bets are finding an audience among sharp players.

Now unfortunately we don’t have a database built that contains halftime lines, so the following research is based on using the full line whole or divided by two to reflect an approximation of a halftime line. Again for the 1st half of games this will be reasonably close, while the second half may in some cases be far off.

Nonetheless, we should be able to glean some patterns to how teams respond to events in the early stages of the game. Our first test is how often the team with the lead in the first half (against half the full spread or straight-up; we’ll look at it both ways) gets the “cover” in the second half against the full line divided by two. In other words, if we treat the second half spread as half the full line (which is an approximation) how often does the team ahead cover in the second half?

So in general, the team that fared well in the first half is unlikely to cover a second half line of “half the full spread” – this is particularly true for favorites. In terms of the actual second half lines, what often happens is that an underdog that’s ahead will be receiving a line that is closer to the full value spread than half of it. Likewise a big favorite that’s far in the lead will not be facing half of the full spread again at that point.

The numbers in the table can also be flipped around to learn the comparable percentage for the team that is behind. For instance an away underdog that’s behind in the first half will “cover” half the full line in the second half at a healthy 59% clip.

The value then seems to be in going against favorites with a lead who are giving at least half the original spread in points in the second half (better of course if they are giving more!). Meanwhile underdogs with the lead can be solid plays if they are getting more than half the original spread in the second half line. There are key numbers in the NFL for spreads (eg 3’s and 7’s) that make it elaborate to give a hard and fast rule as to what the difference in an extra point translates into for a winning percentage. Rest assured though that one or two extra points can usually boost your expected winning percentage significantly!

A more detailed breakout of the above follows, showing the relative lead of the team in the first half --