The Nats’ biggest win of the regular season

Most of us are familiar with “win probability added” (WPA) – a metric which lets you identify which play in a game most contributed to a victory or loss, based on the difference between the probability of winning the game before and after the play takes place. In a close game, the biggest play usually comes late in the game, when a lead is taken or the win is assured. In a blowout, on the other hand, the biggest play may occur much earlier when the team takes an early lead that is never given up.

The same concept can apply to games played—we can look at all of the games played in the season and see how the probability of winning the pennant changes with each game played. In a tight pennant race, the biggest game usually takes place near the end of the season when a crucial game shifts the odds decisively in favor of the winning team. On the other hand, for a race that isn’t so close, such as this year’s NL East race, the biggest game may take place weeks before the end of the season.

I’ve been tracking the probability of the Nats winning the division on the Fangraphs site (though several other sites offer alternative sets of probabilities). Unlike WPA, however, the game-by-game change in the probability of winning the division is not necessarily the most appropriate guide to the biggest games of the season because the change in probability is affected not only by what happens in the game the Nats were playing, but also by what happened in the game played by their rival. (In contrast, for WPA every positive change in probability is matched by a negative change in probability for your opponent).

So I used game-by-game changes in probability to help find some candidates for biggest game of the season, but I combined it with some judgment rather than simply taking the numbers by themselves. I’m pretty confident, though, that the game I’ve identified was clearly the Nats’ “biggest” single game of the season in terms of moving them toward their divisional championship.

On July 7, the Nats began a 4-game series against the Mets in New York. When the series began, the Nats were 4 games ahead of the Mets, and their probability of winning the division, according to Fangraphs, was 77%. If the Mets won the series, they would be right in the race with the Nats, whereas if the Nats won the series, the Mets would be at least 6 games behind. The Mets won the first game, narrowing the margin to 3 games and reducing the Nats division probability to 74.5%.

On July 8, Stephen Strasburg faced “Thor,” Noah Syndergaard, in a matchup of aces. Although Syndergaard had recently been diagnosed with bone spurs, he had pitched brilliantly in his previous start.

Strasburg pitched a superb game, striking out 9 and allowing only two hits and one run in 7 innings, giving up the run on a homer by Asdrubal Cabrera. For the Nats, Clint Robinson hit a two-run homer and Daniel Murphy added an RBI double. Syndergaard had to leave the game in the fifth as his velocity dropped, and Cespedes also left with an injury. The Nats won 3 to 1, and the Nats lead returned to 4 games, with a division winning probability of 83%. Their lead would never again drop below 4 games. It was their biggest win of the season.

The Nats went on to win games 3 and 4 of the series and went into the All-Star break with a 6-game lead. While it isn’t clear whether Strasburg will be able to contribute to the Nationals this post-season, he deserves credit for winning their biggest game of the regular season.