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More home runs are coming

The April numbers tell us we’re only at the beginning of a historical year for home runs.

League Championship Series - Los Angeles Dodgers v Milwaukee Brewers - Game Seven Photo by Stacy Revere/Getty Images

One of the main story lines for Major League Baseball during the 2017 season was the historical precedent set for the home run, as the league home run record was set for a second straight year by almost 500 home runs. Talks of the juiced ball went from beginning to end. The 2018 season saw this story line die down a bit, with the home run total failing to eclipse the marks set in 2016 and 2017; there was even talk of the ball being “de-juiced.”

And now that story line is back to filling time in the baseball discussion list, and rightfully so. There have already been 700 home runs hit this season, more than the month of April in the 2005, 2007, 2009, 2010, 2012 seasons. It seems extremely likely that we’ll see a record amount of balls going over the fence for the first month of the season.

As for HR/FB-rate, we currently sit at 14.2 percent. Dating back to when batted ball-types began to be recorded in 2002, the record for league-HR/FB-rate unsurprisingly came in 2017, sitting at 13.7 percent. With that information, one would think on the surface that it would be a close race for a new record to be set in 2019. But, there’s more...

Backed up with common-sense and numbers, April as a whole is not a good month for home runs. The weather isn’t warm, perhaps the air is more dense, making it harder for balls to travel further in the air. As you can see in the chart below, HR/FB-rate pretty much follows the weather, gradually rising from April to August when it gets warmer before taking a bit of a dip in September.

League-wide HR/FB by month

Month HR/FB
Month HR/FB
April 10.5%
May 10.7%
June 11.0%
July 11.0%
August 11.3%
September 10.8%
2002-2019 FanGraphs

This all matches up almost perfectly with average climate numbers in the United States, as depicted in this visual that shows the average temperature across major cities in all regions.

thefifthpostulate/Reddit

So comparing the HR/FB from this season when 100 percent of the sample size is coming from the number-saturating month of April isn’t leveled analysis. To cancel this out, we can compare this April to past Aprils. Doing this makes the results even more damning. Back in 2017 when the league posted a record 13.7 percent HR/FB, it started out with a 12.8 percent HR/FB in April, a then record, blowing 2016’s rate of 11.8 percent out of the water. From that point on, the league put up a 13.8 percent HR/FB, another high-mark for the combined months of May through September.

And somehow, the current HR/FB stands at 14.2 percent. In April. Not May to September, where again, the record HR/FB-rate for a season in those combined five months is 13.8 percent. It is somehow above that. This means we’re just at phase one for this record-setting pace.

The correlation between HR/FB-rate in April and HR/FB for the rest of the season comes out at r = .83.

This correlation in the data is reliable enough to where we can run a regression formula to give an estimate on what the HR/FB-rate will look like for the rest of the season (May to September). In doing that, the estimate comes out to 14.5 percent, smashing the previous one of 13.8 percent.

The numbers indicate we’re only at the beginning of a historical year for home runs, Whether you like it or not, the home runs are coming.


Patrick Brennan loves to research pitchers and minor leaguers with data. You can find additional work of his at Royals Review and Royals Farm Report. You can also find him on Twitter @paintingcorner.