The first month of the season was marked by cold weather throughout much of the country. It seemed to have an adverse affect on offense, with power numbers particularly affected. MLB players put up an isolated-power figure of .156 this March and April, which was the lowest mark since April of 2016. Rob Arthur, who has performed extensive research on the juiced ball, noticed the ball wasn’t traveling quite as far in early April even after accounting for weather — this despite a barrage of homers in the spring. Alex Chamberlain conducted some research of his own and determined hard-hit balls and barrels weren’t doing as much damage as in previous seasons, and he wondered if baseballs had been de-juiced. It’s an interesting question that deserves further research.
Chamberlain speculated that MLB had taken the juice out of the ball, potentially through the use of humidors. He found that hitters had to hit the ball harder to get it out of the park. He also observed that, when controlling for exit velocity and launch angle, batted balls weren’t quite doing the same damage as in years past. He concluded that, since we are now well past the cold-weather days of April, the change in batted balls this season is meaningful.
To examine the issue further, let’s take a look at ISO by month back to the beginning of 2015, just before the balls changed.
I’ve place an asterisk above the April of each year above to note the start of the season. We can see, in 2015, the effect of a juiced ball emerging in August and September. It should also be noted that April is the least powerful month in every season, and in each of the last two years before this one, we saw a rise in May and again in June before a leveling off for the rest of the year. Although April ISO was lower this season than a year ago, it might have been fair to blame the weather. To that point, we saw a fairly typical jump in May. What we haven’t seen this year is the jump in June and July, when (along with August) ISO is typically highest.
We might speculate that better pitching is the cause for the lack of power this season, but Chamberlain’s research indicates the opposite. We can dig a bit deeper and look at wOBA-xwOBA by month going back to the 2015 season. What xwOBA provides is a general baseline expectation of outcomes based on launch angle and exit velocity. We would expect a low, even negative number in April when the weather is colder because the ball doesn’t travel as far. Balls that might be expected to leave the park stay inside it, reducing the actual outcome below the expected outcome. The graph below generally confirms that line of thought.
In each of the past two Aprils, xwOBA has undershot wOBA by about 10 points, making up the difference later in the year. That information helps to explain why I found that early-season xwOBA had a stronger correlation to rest-of-season wOBA than wOBA itself did when I looked at the numbers a year ago. What sticks out in the graph above is just how large the difference was this April, and while the numbers are getting closer as the season wears on, we are still at a negative figure even as the weather has warmed up. The contrast is even more stark when looking only at Barrels, which this season are the top 7% of batted balls in terms of expected results.
We see the same patterns from 2015 through 2017, but we haven’t had near the normal correction for June — and it has actually gotten worse so far this month, although it is early. That negative number on the barrels is mostly responsible for the large negative number overall. Baseball Savant classifies batted balls in six different categories. In the four weakest categories, xwOBA exceeds wOBA by just 7 points. On solid contact, the level below Barrels, the difference is 14 points, and on Barrels, the difference is 149 points. Despite comprising just 13% of batted balls, Barrels and solid contact account for 76% of the difference between xwOBA and wOBA this season. The better contact a player makes in 2018, the more likely he is to feel suffer the influence of this “de-juiced ball” effect.
To provide a comparison, the graph below includes both ISO and xwOBA-wOBA on Barrels since the beginning of 2016. Note that the highest month for ISO in each of the last two seasons has been June.
Unlike the previous two seasons, ISO has been highest in May this year. Based on these trends, those expecting a homer barrage this summer might be disappointed. April was always going to be a modest month for power, and an early start to the season plus unseasonably cold weather provided cover for the big power drop. It turns out that drop was much bigger than should have expected. While we are seeing a somewhat normal correction over the past few months, that correction is still leading to results well below what we should expect based on the quality of the contact.
Whether this is the result of humidors or some subtle change to the ball, that isn’t clear from this data. What does seem clear, though, is that the ball isn’t traveling as far. A year ago, when a batter made recorded either “solid contact” or a Barrel on a fly ball from May 1 through July 12, the ball traveled 392 feet on average. This season, from May 1 to July 12, on the same type of contact, the ball has traveled just 385 feet. That’s a big difference — and a big part of the reason why batted balls traveled out of the yard 4.9% of the time last year and just 4.5% of the time this season. That figure is still in line with 2016 and higher than pre-2016 figures, but it is a pretty clear reversal of a trend that had been ongoing for the last few seasons. The ball isn’t dead, but it might not be as lively as it was the past few seasons.
Craig Edwards can be found on twitter @craigjedwards.