The American League Resumed Interleague Play Dominance in 2021

For nearly a decade, you couldn’t go a week in the offseason without seeing an article about the American League’s dominance over the National League in interleague play. I know – I was already a rabid consumer of Hot Online Baseball Takes™ at the time and drove myself to distraction trying to find reasons to believe or disbelieve it. But it was interesting! The AL and NL split World Series roughly down the middle, but the AL kept winning interleague play and World Series games. Was it just a better league?

In 2018 and ’19, the trend flipped. The NL won the interleague season series in both of those years, which marked their first victories in that theoretical season-long series since 2002 and ’03. It had been quite a while, in other words.

By 2020, “who’s winning the interleague series” didn’t feel as interesting, and the unique pandemic-shortened schedule drove that point home even more surely. Due to geographically-divided schedules, there were nearly as many interleague games in 2020 as there were in ’19 (298 league-wide as compared to the standard 300 on the schedule every year since 2013), despite playing a 60-game season rather than 162. Not only that, but there wasn’t the usual rotation of opponents and rivalries. Instead, each division played its opposite-league counterpart.

The aggregate league records in 2020 didn’t make much sense, because the NL and AL only existed in theory during the regular season. The Dodgers and Braves, who met in the NLCS, operated in completely different bubbles until the playoffs. As if that weren’t enough, there was no signal there, either – the season interleague series ended tied, 149-149. Good luck finding any meaning in that.

All of which brings me to something I didn’t know until Ben Lindbergh pointed it out on Effectively Wild – the AL was back on top in 2021, winning 167 of the 300 interleague games, a .557 winning percentage. Is the old state of the world returning? Is this just a one-year blip? I thought I’d do a little digging and see whether there was anything interesting about this year’s reversal.

Many of the normal things you would do to contextualize records don’t work all that well when you’re thinking about the league as a whole. For example, let’s say you wanted to work out whether one league had an easier “strength of schedule.” Strip out the interleague games, work out each team’s record not counting those, aggregate across the entire league – and naturally, both the AL and the NL sent teams with an aggregate .500 record to the interleague gauntlet this year.

They also do that every year. If there were no interleague play, both leagues would obviously end up with an aggregate .500 record – there has to be a winner and loser in every game, after all. As long as each team plays an equal number of interleague games – and they do – league record is meaningless. The same is true for whatever advanced record estimator you want – again, every game has a winner and a loser, and so the overall record of a league is constrained to .500.

Instead, I looked at what happened in the interleague games themselves. Run differential is a decent proxy for how well each side played – particularly over 300 games. Just as you’d expect from the raw results, the AL outscored the NL last year. It wasn’t by a lot, though: they scored 4.61 runs per game and allowed 4.49, which equates to a .513 Pythagorean expectation. In other words, the American League got the best of their NL counterparts, but by far less than you’d assume if you merely looked at the record. The Yankees were the biggest culprit here; they were actually outscored by 15 runs in interleague games, but went 12-8 (they split six games against the Mets while being outscored by 15 runs).

I was curious whether these results were a fair reflection of each team’s skill, so I checked to see whether the players who had played in these interleague games roughly matched the league’s overall talent. I took a weighted average of the season-long wOBA allowed of each league, weighted by batters faced. Then, I used those season-long wOBA numbers but with new weights: the number of batters they faced in interleague games.

The idea behind this was to see whether one or both leagues were disproportionately using their best (or worst!) pitchers. If the AL started mainly aces and the NL started mainly scrubs (relative to how often those pitchers started overall), you might expect that to tilt the results.

There was little effect to be found, however. The NL used pitchers who were in aggregate slightly better than league average – with an emphasis on slightly. NL pitchers allowed an aggregate .313 wOBA in all games. The weighted average of the pitchers who threw in interleague games checked in at .311. That’s a tiny difference – basically nothing.

The AL’s interleague pitchers were even more representative – overall .316 wOBA, interleague-weighted .315 wOBA. In other words, both teams sent out a fair sample of their pitching to face the other league. The same was true for hitters on both sides, after stripping out the DH – I wasn’t exactly sure how to handle DHs, but ignoring them seemed like at least an acceptable outcome, and this likely won’t come up after this year.

Okay, so both sides brought representative teams to the AL/NL clash, and the AL won. Does that mean anything?

My first instinct is that it doesn’t. The results of the Tigers-Pirates season series (Pittsburgh won 4-2, for what it’s worth) simply don’t matter if we’re wondering which league is “better.” I also don’t care whether the Rays go 5-1 or 6-0 against the Marlins (they went 5-1). If we want to know who’s better, why not try focusing on teams that might compete for a playoff spot?

Any cutoff is going to be arbitrary, but I decided to look only at teams who won at least 75 games in the 2021 regular season. That excluded 10 teams – six NL squads (Diamondbacks, Pirates, Nationals, Marlins, Cubs, and Rockies) and four in the AL (Orioles, Rangers, Twins, and Royals). Then, I went through the seasonal records with every game involving one of those 10 teams removed.

Nothing changed! The AL went 73-61 in those games, a .545 winning percentage. If “how the teams that are trying do against each other” is a relevant metric, it still looks like the AL is better. Unless you define trying differently, of course – you could cut out teams that didn’t reach 80 wins, or that didn’t make the playoffs, or whatever bright line you prefer, and maybe get a different answer.

Oh yeah, one more thing. The AL edge? It’s actually an AL East edge. In games between the four competitive AL East teams (Boston, New York, Tampa Bay, and Toronto) and the three competitive NL East teams (Atlanta, New York, and Philadelphia), the AL went 35-15. The entirety of the AL’s edge, in other words, came from that absolute dominance.

That doesn’t make it less valid. It does, however, call the meaning of the whole exercise into question. The schedule this year was once again tilted toward divisions facing their geographic counterparts – West played West, East played East, and so on. Sure, we quote “interleague record” like it’s some cohesive thing, but it’s really not. We didn’t get a chance to see the Rays take on the Giants, or the White Sox clash with the Dodgers, or whichever matchup you think might indicate which league had an edge on its counterpart.

Meaningless conclusion? Indubitably! The AL East was a lot better than the NL East, and it was by enough to make up for how poorly the AL West did against its counterpart. I’m not sure that interleague record ever meant anything, and I’m not sure that it does now, but to the extent that it’s a thing, congratulations to the American League on restoring the state of interleague play that held from 2004-17. And hey, while we’re at it, congratulations to the National League – they went 7-7 in the World Series in that 14-year span. I’m not sure that’s the first definition of parity that springs to mind in baseball, but it’s certainly amusing.