Over the weekend, I attended the Sloan Sports Analytics Conference, and on Friday, I attended a couple of the presentations of research papers that were baseball-oriented. The first one, presented by John Salmon (based on work he did with Willie Harrison), was titled “Bullpen Strategies for Major League Baseball“, which is obviously a hot topic in the game right now. But while the title made the paper sound like it might touch on Andrew Miller’s postseason usage, the part of the talk I found most interesting actually had more to do with starting pitchers.

In the first half of Salmon’s presentation, he talked about home field advantage, and particularly, how the data shows that one of the primary factors in a home team’s edge might be that their starting pitcher just takes the mound first. While Jeff Zimmerman wrote about this here on FanGraphs back in 2013, I must have missed that piece, because this was new information to me. But Salmon is correct that the data is clear that the gap in performance between the home and road team in the first inning dwarfs the difference in every other inning.

1st Inning Starting Pitcher Performance Team 1st Inning All Other Innings Home 0.323 0.320 Road 0.347 0.330 Difference -0.024 -0.010

This table covers data from 2002-2016, but in recent years, the first inning gap has been close to 30 wOBA points, while it drops to about 10 wOBA points in innings after the first inning. According to Salmon’s calculations, this huge difference in starting pitcher performance means that the first inning accounts for about 21% of a home team’s overall advantage.

So while a lot of the prior research done on home field advantage has focused on ballpark familiarity, the comfort that comes from sleeping in your own bed, or even subconscious umpire bias, Salmon suggests that perhaps a good chunk of why home teams win 54% of the time is that the away starting pitcher has to sit in the dugout and watch his teammates hit after finishing his pre-game warmup.

Zimmerman’s post from 2013 includes a couple of graphs which seem to support the point.

Jeff’s research showed that road starters generally throw a little harder than home starters in every inning besides the first inning, but that effect is not true in the first inning, which could be consistent with some kind of cool-down effect. He also showed that first inning fastball velocity declines even more if the road team builds a big lead in the top of the first inning, which of course would require even more time for the starter sitting on the bench after finishing their warm-up throws.

This isn’t a smoking gun or anything, and since this was just a portion of Salmon’s presentation, he didn’t dive too much into the possible alternate explanations for the first inning performance differential, or prove definitively that it is because of a cool-down effect. I did run the idea by a couple of Major League pitchers, though, both of whom said they could buy that the gap in time between warm-ups and actually throwing their first pitch could be part of the reason that road pitchers tend to do relatively worse in the first inning.

If the cool-down effect was indeed the main cause of this performance gap, then Salmon had a potential solution; road teams should simply begin the game with a reliever on the mound, preferably one who was a good match-up for the home team’s leadoff hitter. The reliever would absorb the penalty associated with finishing his warm-ups and then not pitching for a while, but this could potentially be offset by gaining the platoon advantage and using an arm who would specifically do well against the guy you know is hitting first. Thus, Salmon suggests, the starting pitcher could finish his warm-ups during the first batter at-bat, and come straight into the bottom of the first inning without any cool-down period.

The people from Major League teams who were also at the presentation who I talked with called that idea unrealistic, given that it forces you to burn a reliever for just one batter early in a game, limiting the ability to get specialist match-ups in late-game, higher-leverage situations. But even that part of the suggestion seems unnecessary, at least in the American League, where the starting pitcher could theoretically push his warm-up back a bit, so that his routine finished during the top of the first inning. This probably wouldn’t work as well in the NL, where a starting pitcher might have to bat in the first inning occasionally, but AL teams don’t have to worry about that complication.

This could also have some potential impacts on how teams handle things in September, when pitching staffs expand, and the cost of burning a reliever in the first inning isn’t nearly so high. This kind of potential gain might not be large enough to incentivize change in April through August, but if you’re a contender fighting for a playoff spot in the final month of the year and you have access to basically unlimited pitching changes due to the expanded rosters, it might be worth it to try something different in order to gain an edge that could help save you a run in a game that could be the difference between making the postseason and watching at home.

And then, there is October. With frequent off-days and the highest stakes possible, teams are already experimenting with wildly different pitching usage plans in order to gain any advantage possible. If road teams could significantly improve their pitching performance in the first inning by simply using a reliever or two to handle the first inning, and then bring in the starter to begin the second inning, that’s a little less crazy when the impact of a single win or loss has a dramatic impact on your likelihood of bringing home a championship. In the winner-take-all Wild Card game, for instance, teams should already be unloading their bullpens, and that game might be where this idea has the most benefit.

Of course, we don’t know for sure that the difference in first inning performance is due to this time between warm-ups and taking the mound, and teams generally prefer not to mess with routines to maybe gain an advantage of unknown size. I wouldn’t expect this to become a trend any time soon, but when the Wild Card games roll around, it will be interesting to see if the road teams decide to mix things up a bit in an attempt to diminish the home team’s large first-inning advantage.