Quantifying Momentum in Major League Baseball
Photo Credit: Seth Wenig/AP Photo
The team that scores first in the MLB wins 69% of the time. That number may feel surprisingly high for a sport that averages just under 4.5 runs per game per team. Why is taking and keeping a lead in baseball so vital?
To address this question, I analyzed all pitches since 2015 and filtered for those with runners in scoring position (RISP). I focused on RISP situations because they capture the pressure and high leverage associated with run-scoring volatility. I then grouped these RISP pitches by score difference (batting team’s runs minus fielding team’s runs) and calculated the average xwOBA for each score difference. Figure 1 illustrates my findings.
Figure 1: Quadratic relationship between xwOBA in RISP situations and score difference
As we can see, the dataset closely follows a quadratic distribution, with xwOBA with RISP reaching its minimum when a team trails by 4.4 runs.
Here’s my best explanation for why hitting with RISP and a 4.4 run deficit is particularly challenging:
When the score difference ranges from -5 to -1, opposing teams are often bringing in their better or even best relief pitchers to try to nail down the win.
When trailing by 4 or 5 runs, the batter really feels the pressure to deliver with RISP, and on average performs worse under that added pressure. He knows that if his team doesn’t come through in this situation, they may not have many more chances to come back in this game.
However, when trailing by more than 5 runs, opposing teams are more likely to bring in lower leverage relievers. As a result, xwOBA with RISP improves as the deficit grows greater than 5 runs.
In a close game, every score difference improvement results in higher xwOBA with RISP. As the score difference moves in favor of the hitting team, pressure on the offense is released and pressure on opposing pitchers grows, causing them to perform worse.
As score differences increase from -1 runs, opposing teams pitch worse and worse pitchers on average, and thus xwOBA with RISP starts to increase even more quickly.
