Nick

There is lots of evidence that the HR environment is different since the All-Star Break, but the exact cause is still unclear (long post, not AI, tldr at the bottom) Usually when there's a significant change to the baseball, it shows up in distance/flight/drag data. While you can make a case the ball hasn't been flying as far as expected, that *is not* the main reason this time. The clearest signal is that contact quality has suddenly gotten worse. Take a look at the chart below (14-day moving average). Expected-HRs-per-Ball-in-Play dropped out of normal variation right around the All-Star-Break. So what's in this number, and what does it mean? It means: "Based on the physical characterstics of how the balls are getting hit (exit velocity, launch angle, direction), we should expect FEWER batted balls to result in home runs." In other words, if you ONLY look at how the ball is leaving the bat as soon as it's hit, there are fewer batted balls that SHOULD end up going over the fence. From there the analysis gets really messy. Unlike distance, there are wayyyy more factors that can contribute to worse contact quality (specifically worse HR quality). - Exit velocity, launch angle & direction - Pitch location, movement, velocity, shape, spin - Bat speed, plane, angle - Batter's approach / skill level This is why it's hard to say for sure that a ball change occurred. But there are some general assertions that can be made, through analysis not shown here: - Bat speed has not gone down - Pitch velocity has not gone down - Neither pitch quality nor pitch mix has changed in a way that should produce fewer home runs. - Summer weather is still having a positive HR effect on the balls getting hit From MLB's Drag Dashboard on Baseball Savant: "The performance of the baseball is primarily driven by two factors: the coefficient of restitution (“COR”), which impacts the exit velocity of the ball off the bat, and the drag coefficient, which impacts how far a batted ball carries once struck. All else equal, an increase in COR will lead to greater exit velocities, while an increase in drag will decrease the carry distance. Together, these two properties have a significant impact on the run environment in Major League Baseball and on the home run rate, in particular." Logically, and through analysis not included here, the best guess appears to be that a new ball was introduced around the All-Star Break that caused a decrease in COR, leading to slower exit velocities. This is only a guess at this point, because of how complicated exit velocity is. But it does seem to be the best guess, and is the assumption Ballpark Pal is operating under in its projections. TL;DR: The post-All-Star-Break drop in home runs appears to be driven more by worse contact off the bat than by reduced carry. Bat speed, pitch velocity/mix, and weather don’t explain the change. The hypothesis that Ballpark Pal is operating under is that a new baseball with lower COR - and slightly more drag - was introduced around the break, reducing exit velocity and HR-quality contact. That fits the data, but it is not proven.