Death of a 14-Year Streak

Monday evening, the Pirates beat the Royals, 7-6. The Royals were up by two in the bottom of the eighth, but the Pirates rallied to tie, and then they walked it off an inning later. With Kevin Kramer leading off second base, Jacob Stallings sent a low line drive into left field, and Kramer beat Alex Gordon’s throw home. The Pirates rushed out of the dugout to celebrate the victory:

Now, this season, the Pirates are going nowhere. The Royals are even worse. The win did, I suppose, push the Pirates back over .500, but it’s worth remembering that every game is a competition. Every game features major-league baseball players trying to win, and so every actual win legitimately feels like an achievement. Especially for teams full of players just trying to make a good impression to extend their careers. Recently, the dreadful Royals walked off against the dreadful White Sox, after a throwing error on a would-be sac bunt. The Royals celebrated in regular fashion:

So part of this is just that a walk-off is fun. You never know when you’re going to be part of another. Seize happiness; it’s fleeting. Celebrate your achievements. Few people in the world ever get to participate in a win in the major leagues. What an incredible thing it is to experience.

There’s something else about the Pirates’ victory, though. None of the players would’ve known it at the time, but you can see their celebration as symbolic if you want. The win was the Pirates’ 75th of the season. But it was also the National League’s 151st win in interleague play. Every year, every team plays 20 interleague games. That means there are 300 interleague games in all. And for the first time since 2003, the American League isn’t going to win the majority of them. It won’t win exactly half of them. Interleague play, in 2018, belongs to the senior circuit.

I wrote about this as it was developing back in June. But, back in June, when I first took a look, the interleague schedule was only 39% complete. More than that, the AL side had been skewed by some of its worse teams, and the NL side had been skewed by some of its better teams. In other words, the data then was interesting, but there was a lot more left to play. That’s not the case anymore. The regular season is just about finished. There are but nine more interleague games to play, and, on the AL side, they’ll all be played by the Royals, Tigers, and White Sox. So we can go through identical data plots. This post is an update of the last one, which is convenient for me, because it means I have to be minimally creative.

All interleague data comes courtesy of the Baseball Reference Play Index. I’m going to show you three images. The first one is the simplest one — here is AL interleague winning percentage, stretching back to 1997:

For 14 straight years, from 2004 through 2017, the AL finished over .500. Obviously, some years were more dominant than others, but the NL never quite had the edge. Last year, the AL won 53.3% of the games. This year, it’s down to 47.8%. If the AL wins out — which is unlikely — it will have an interleague winning percentage of 49.3%. If it loses out, it will be 46.3%. The worst year for the AL was 1997, the first year of interleague play, when the AL won just 45.3% of the time. This year won’t be quite that lopsided, but what’s most important here is just the general shift in the balance of power.

Now, anyone who hangs out at FanGraphs knows there’s a little more than just overall winning percentage. Sometimes, even over 300-game samples, winning percentage can mislead. Going to the next level, let’s look at the year-to-year run differentials per game:

Same idea here. This is like a proxy for looking at the Pythagorean record. From 2004 through 2017, every single year, the AL outscored the NL. Last year, on a per-game basis, the AL outscored the NL by 0.35 runs. This year, the NL has the advantage, by 0.22 runs. That would stand as the second-greatest NL advantage on record, behind only 1999. So the winning percentage isn’t deceptive.

One last plot now. Our first looked at winning percentage, which is first-level stuff. Our next looked at run differential, which is second-level stuff. Now let’s look at OPS differential, which is third-level stuff. Run differential mirrors Pythagorean records. OPS differential mirrors BaseRuns records. I recognize that OPS isn’t quite as effective as wOBA, but the two numbers move in parallel, and with differences like what you’re about to see, it’s hardly worth the extra effort to run manual calculations.

Here we see something that’s even more striking. The AL actually had the OPS edge for 15 straight years, including 2003. Last year, the AL out-OPSed the NL in interleague play by 33 points. This year, the NL has out-OPSed the AL in interleague play by 26 points. That’s never happened before — the biggest NL advantages have been by 16 points, once in 1997 and once in 1999. Technically, because the interleague schedule isn’t entirely over, these numbers still have time to shift, but at least by this one measure, the NL looks stronger than ever, relatively speaking. I know the ultimate goal is to win, and not simply to out-OPS your opponent, but this is fairly convincing evidence that the NL has taken a significant step forward.

You might figure that the top of the AL is superior, but that the league as a whole is brought down by everyone else. That suspicion would seem to be accurate. The five current AL playoff teams have a combined interleague winning percentage of 64.0%. The rest of the AL is at 39.3%. Meanwhile, the five current NL playoff teams have a combined interleague winning percentage of 55.3%. The rest of the NL is at 50.8%. It’s not like every single team in the AL is suddenly inferior to its nearest NL equivalent. But I’m not interested in getting that granular. For one reason or another, the AL overall has lost its advantage. The fact that it won for 14 straight years makes it notable that, this year, it won’t.

I don’t know yet exactly what it means, and this could always just be a blip. The AL could regain its superiority in 2019, and maybe a new streak will begin. All these rebuilding teams have no intention of staying bad forever. For now, we can just officially kiss a long streak goodbye. All those popular theories that the AL gets some kind of structural interleague advantage — I suppose this doesn’t disprove them. But it does show that, if advantages do exist, they’re not insurmountable. Congratulations to the National League. If only you could all celebrate this victory together.