You may have heard that it is never correct to hedge a pending bet when the hedge itself is a -EV play. This is usually good advice, but not always.
The most common situation where a bettor wants to hedge is when he has hit every leg of his parlay with one game to go. Hedging here makes no sense: you should have simply left this game off your parlay and taken the lower payoff for one less game. People often don't realize until it's too late that their $5 parlay can leave them with over $1000 riding on the last game. Technically, it may be better for your bankroll to hedge this last game than let it ride, but you never should have been in this situation in the first place. If you can't handle this swing and will want to hedge the last game, either don't bet parlays or bet smaller ones.
That said, there are a few situations where hedging can be correct. The common link is that you have already made a bet with a relatively large payoff, and it's better for your
long-term bankroll growth to reduce your risk somewhat. In other words, you want to hedge when doing so will increase the geometric mean of your possible outcomes, bringing you closer to the optimal strategy for bankroll growth. (For more on this concept, I recommend the excellent book
Fortune's Formula.) A few of these situations aren't common, but there is one that occurs quite frequently, which we'll get to later.
Let's say that you're offered a $10 freeplay bonus, and it can only be used to parlay every NFL game on the board in a given week. Unlike the previous example, here you don't have the choice to parlay fewer games, so a hedge can make sense. If you've gone 15-for-15 so far, you should certainly hedge the Monday Night game unless you are already quite wealthy. In fact, you should probably hedge before the Sunday Night game. You're giving up 4.5% of your EV, but that's not a big deal when tens of thousands are at stake. (Remember, this is only when you aren't able to simply parlay less teams and collect the smaller payoff without paying extra juice.)
David Sklansky outlined a couple of other examples in one of his books. Let's say you want to bet the Brewers to win the NL Central at 9-1. You can't find anyone offering these odds, but you do see 35-1 on the Brewers to win the NL Pennant. If they make it to the playoffs, you estimate you can take an average line of -350 on them to NOT win the NL.
If the Brewers make it to the playoffs either as a Wild Card or the division champ, you can hedge the bet for the full amount, risking 28 units to win 8 and guaranteeing a 7 unit profit. In effect, you're getting 7-1 on the Brewers to make the playoffs. It's not quite 9-1, but you also collect when they win the Wild Card, so it's close enough. Even if you suspect the -350 hedge is somewhat -EV on its own, this combination of bets is well worth making if you think Milwaukee will make the playoffs 25% of the time. The hedge allows you to risk a larger portion of your bankroll on the NL Pennant bet than you would otherwise, since it will not be a disaster if the Brew Crew lose in the playoffs. It is this ability to risk more of your bankroll, and thus increase your overall EV, that makes the hedge a valuable tool.
Another example: Suppose that you know a stupid but honest bookie who is dealing a +150/-160 moneyline on a game when everyone else has it at +120/-130. You estimate the correct line at +110/-110. Assuming that your bet won't get no-actioned, you clearly want to make a max bet with your bookie. However, this maximum may be a large fraction of your bankroll, far above the recommended Kelly bet size. In this case, your best play is to max out your bookie, but take the -130 for a partial hedge/arb. This gives you the equivalent of a better line than +150 on your overall bet, and doesn't put your bankroll at too much risk. (This is similar to another Sklansky example where you play a middle in football rather than just one side, because you can bet larger amounts that way with little risk.)
A similar situation comes up all the time in sports betting. You bet a team heavily at +150 against an estimated correct line of +100, wagering the correct amount for long-term bankroll growth. Close to game time, the line has moved to +120/-130. Is it correct to hedge part of this bet by laying -130 on the other team?
Absolutely it is. Assuming you sized your first bet correctly, it will always be better for your long-term bankroll growth to hedge a fraction of this bet. If you don't believe me, calculate the geometric mean of your possible outcomes before and after hedging $1 of your bet. It will improve after the hedge every time.
An example will help clarify. You have only one bookie, but he lets you bet juice-free; each game's moneyline looks like +125/-125. Furthermore, you have a perfect estimate of the correct line for each game. With this information, you can calculate optimal bet sizes for every game. The optimal bet size is something of an equilibrium for a given game; after you bet that amount at +150, you're essentially indifferent to wagering an additional $1 at +150 or hedging $1 back at -150. However, if you are offered better odds than +150 or -150, this shifts the equilibrium point, and you would now place additional bets to restore equilibrium. (If that doesn't make sense, try re-reading it. Those who did well in high-school science classes will probably better grasp the equilibrium concept.)
What happens if the line on this game suddenly moves to +130/-130? Now you have a strong incentive to hedge, even if the game is a coin flip. Remember, you didn't want to add to your bet at +150. If you don't want to subtract from it at -130, that essentially means you are inclined to bet more at +130, or you're at least indifferent to doing so. Something's gotta give: Either you should have bet more at +150 or you should hedge a portion of your bet now. Since we have established that you bet the correct amount to begin with, the hedge must be correct.
I'm guessing that plenty of you have encountered this situation following these picks. Yesterday's Florida-St. Louis game was an example. I advised a hedge at StL +107, even though I believed that they were slightly bigger dogs to win than this. But for the purposes of long-term bankroll growth, it was best to hedge most of the bet on Fla +102.
In general, it will always be correct to hedge part of the bet when the line moves this much. However, I've yet to find an easy function to calculate what hedge amount is best, so that's for another post.