Thus far, the Pirates have gone 18-14 against their own division while the Diamondbacks have gone 21-11, each with 20 games left to play. Here’s the full breakdown of games left:

Pirates: 7 vs. MIL, 3 vs. CHC, 6 vs. STL, 4 vs. CIN (6 home, 14 away)

Dbacks: 3 vs. LAD, 7 vs. SDP, 4 vs. SFG, 6 vs. COL (10 home, 10 away)

So in that subset, with a clearly harder schedule, the Pirates would have to make up 3 games on the Diamondbacks. And for the tiebreaker to matter, the Diamondbacks would have to also make up at least 1 game in the 33 extradivision games each have left, where the Diamondbacks have a clearly harder schedule (I’ve sorted by current record and crossed out the common opponents for clarity):

Pirates: 3 vs. LAD, 3 vs. CWS, 3 vs. BOS, 3 vs. SDP, 3 vs. MIA, 6 vs. DET, 3 vs. NYM, 3 vs. SFG, 3 vs. KCR, 3 vs. LAA (18 home, 15 away)

Dbacks: 3 vs. ATL, 3 vs. NYY, 3 vs. CHC, 3 vs. PHI, 3 vs. BOS, 3 vs. TEX, 3 vs. CLE, 3 vs. MIA, 3 vs. HOU, 3 vs. CIN, 3 vs. KCR (18 home, 15 away)

These sort of surprising combinations of results can happen, of course — the Pirates bookended their first half by losing a series to the Mets and sweeping the Brewers, for instance — but they aren’t likely, which is why I have the Diamondbacks clearly favored in that tiebreaker.

Of course, all of this is entirely irrelevant if the Pirates win the head-to-head tiebreaker. And the Pirates are the main wildcard competitor for that third spot right now. Fangraphs puts the Diamondbacks’ current playoff odds at 38%, the seventh-highest in the NL, immediately behind the Pirates at 40%. Those numbers have each jumped by about 5% with each game this series, which is a lot for games in July.

All this to say, the next game is very, very important.

Here’s the real list:

Head-to-head

Intradivision record

Intraleague record

Post-ASG intraleague

Post-ASG + 1 game intraleague

Post-ASG + 2 games intraleague

etc.

This will always eventually resolve, because different teams play different number of interleague games at different times.

The good news (for thinking through this, not for the Diamondbacks) is that in the four years of this system so far, no ties have ever made it past head-to-head!

As a follow-up, I also asked ChatGPT to analyze all 11 ties in the divisional era (1969 onwards) that went to game 163’s before the modern tiebreaker rules were in effect. Out of all 11, 10 would have been resolved by head-to-head, while only the 1980 NL West tie between the Dodgers and Astros would have gone to the intradivisional record.

So yeah, this tiebreaker I analyzed extensively would only have mattered once in the last 50+ years…