Home field advantage is no big deal

Yesterday the Red Sox finished one race in first, but fell a step behind in another. With just four games to go, the Sox are now a full game-and-a-half behind the Texas Rangers for the top seed in the American League—a gap they seem unlikely to close as they get their starters rested up and set for the postseason.

In an ideal world, the Sox would get the number one seed, have home field advantage, and be afforded the right to choose their ALDS opponents by virtue of that top seed. Of course, that’s not how it works. Top seed faces the wild card winner, second seed faces third seed, no matter how good the competition might be.

We’ve been over the effect this has on Boston’s incentive to actually win in these last few days. The Indians are a team that’s been knocked down hard by injuries to their rotation. They are still a playoff team, but at least for now look like a tragic case. Saying that the Red Sox would rather face them than, say, the Jays is nothing against the Indians and what they’ve done this season. They’ve just been on the wrong side of luck at the most important time of the year, making them any playoff team’s ideal opponent.

Still, some seem to think that facing a better opponent is a reasonable trade-off if it means taking home field advantage. But how much of an advantage does that actually confer? Very little, it turns out.

A survey of postseason results will show that the home team does win more often, but this is to be expected. Yes, partially because teams are better at home than on the road, but also because there’s a selection bias going here. The home team is more often going to be the better team simply because of how home field advantage is decided in the first place. Team quality does not perfectly correlate to season record, but over samples of many years we’ll find that teams that win more games are better.

Instead, let’s look at what the home - road difference has actually meant to the teams in question. As a warning, the math here is fairly dirty. It’s not meant to produce actual odds for a series between the Red Sox and team X, Y, or Z—only to demonstrate that shifting the venue for one game in a series promises no seismic change.

For the Indians, it’s actually been pretty big. They’re a game under .500 on the road after last night’s loss, and 53-28 at home. For the Rangers, it’s still a large gap, if slightly less so—52-26 at home, 42-39 on the road. The Red Sox, on the other hand, are fairly flat: 46-32 to 46-34.

So let’s take the extreme example in Cleveland, and say the Sox actually fall behind them for the #2 seed and have to play the ALDS on the road. Going by Cleveland’s numbers, we’re looking at three games with a 67% chance to win and two with a 49% chance to win. Average it out, and the Indians come away with a 58.6% chance to win (not the series, but a game).

That sounds bad for Boston, but it’s because we’re looking at the Indians’ overall chances of winning against a random team. What we should really be comparing that number to is their chances if we were to switch home field advantage. Now it’s three at 49% and two at 65%. That leaves them with an average 55.4% chance to win. A 3.2% chance swing per game isn’t nothing, but it’s also not huge.

And this is the biggest possible way to interpret the gap, because this is not the Indians vs. any average team, but the Indians vs. the Red Sox. Looking at Boston’s numbers, the Sox would expect a loss of home field advantage to hurt them very little. With home field, the Sox average out to 58.4%. Without? 58.1%.

The reality lies somewhere in the middle, very likely. The loss of home field advantage shifts Boston’s average chance of victory a percentage point or two. This change is effectively stacked together in the one game that is held in Cleveland rather than Boston, granted, but at the end of the day it’s just one game in five, and only one middling aspect of that game actually changing.

That advantage is diluted further if the Red Sox move on to the ALCS. There that one game is just one in seven, and the opponent—if, indeed, the Sox lack home field advantage—one that hasn’t displayed quite as significant a difference in their home and road results. Just to run the dirty math again, we’re looking at 60.5% with home field and 58.4% without. Over more games, granted, but still a smaller impact. The gap between Boston’s numbers shrink by a tiny amount, given that there was barely any room to begin with.

That’s not, as warned, exactly what the Red Sox stand to gain or lose over these last few days. But it does give you an idea of the magnitudes we’re dealing with here. These are small shifts in the team’s chance to win the series. Now weigh that against their chances of beating an Indians team that’s missing two of their three best starters compared to their chances of beating the Jays or O’s or Tigers. If they had been without Carrasco and Salazar all year, these Indians might not have even been in the Wild Card race. On the other hand, the Jays in particular, seem quite the threat given that Baseball Prospectus’ adjusted standings suggest they’re the second-best team in the American League after these Red Sox.

Yes, whoever comes out of that game may well only get one start from their ace if they have to use them up in the Wild Card round, but the Blue Jays are a team with multiple solid options rather than one great starting pitcher, and the Tigers have Fulmer to go with Verlander. The Orioles are the team most likely to get hit hard by that inconvenience, and they’re built more around the bullpen than the rotation to begin with.

Realistically, home field advantage in the ALCS will not have nearly so large an impact on the Sox’ chances of success as their opponent in the ALDS, and will only even be a factor if the Red Sox and Rangers both make it through their first round. Against the Indians, the Sox should certainly try to bring the series to Boston as much because that’s where it’s likely to help them most as because there’s just no downside to it. But if they choose to rest up, avoid injuries at all cost, and risk heading to Cleveland rather than opening in Fenway, we should be neither surprised nor upset. It’s a reasonable tradeoff to make.