Rookies Are Keeping Their Minor League Power

As a Royals fan, I had my doubts Jorge Bonifacio would be a major league contributor. Over the past couple of seasons, my opinion has changed as he showed some power in AA in 2015 (17 HR) and AAA in 2016 (19 HR). I fostered some reservations on the AAA power because he played in the offensive happy Pacific Coast League.

He’s started 2017 off great with 3 HR and a 10% HR/FB ratio in AAA before getting his major league call-up. Since the promotion, he’s hit six bombs with a 29% HR/FB rate. Owners may be expecting some heavy regression from Bonifacio but they shouldn’t. The “juiced” ball era has reversed a trend of position players hitting for less power once getting a major league promotion.

Fans and writers noticed the significant home run increase starting in the middle of 2015. Here are the HR/FB and HR per 600 PA rates since 2002.

Home runs are up. Why they are up is being debated. I’m not going to waste paragraphs on the current stance on the increase’s source. For about one and half seasons, the only explanation was a juiced ball. In the last few weeks, two articles have been written debunking the juiced ball. The increase is real and continuing in 2017.

With major league home runs increasing, here are the AAA home run rates for comparison.

While high, the 2017 HR/FB rate still not as high as the 2011 number. Also, the 2017 HRs per 600 PA rate has been beaten several times. There is no home run explosion going on in AAA.

Now, how do hitters compare when they spend time in both AAA and the majors? To find out, I compared the home run rates for hitters who played in both AAA and the majors between from 2008 to 2017. I weighted each player’s home run performance to the harmonic mean of their plate appearances. As for survivor bias, there should be little because over performers in AAA will get promoted to the majors and then regress down. On the other hand, underperforming hitters in the majors will get demoted to AAA and then regress up.

Here are the results.

Most fantasy owners probably expect what happened from 2008 to 2014. Hitters lost some power with the transition to the majors seeing their season-long home run total adjust down by five. Additionally, their HR/FB dropped 2.5% points.

Then the past three season happen. The ~4 HR per season increase is significant. The big difference is the 3% increase in HR/FB rate. The HR per 600 PA has the influence of more strikes outs possibly limiting it jump. With HR/FB rate, owners should expect hitters making the jump to the majors to at least keep up the rate or improve a small amount. Additionally, hitters getting demoted, may not see their power numbers jump.

A juiced ball seems like the most obvious answer to the jump but fantasy owners shouldn’t worry about the exact reason (unless they really want to). Many of the smartest minds are trying to find an answer. Let them do their work. What we do know is:

1. Home runs are being hit at an all-time rate
2. There doesn’t seem to be a home runs drop between AAA and the majors.

I also wanted to know how projections were handling the adjustment. I contact Dan Symborski, the creator of ZiPS, and he said he has taken the change into account. He separates out the AAA translations based on league but only adjusts the translations between seasons.

“… out and from the 1418 International League homers in 2016, the combined translation for them in the majors was 1445 [1.02 multiplier]. The 1932 Pacific Coast League homers became 1887 [0.98 multiplier].”

As for Steamer, it’s creator Jared Cross stated he hasn’t made any adjustment yet for the “juiced” ball. He said too much noise exists already with AAA-to-MLB translations to make any major changes but he is looking into future changes.

Using the historic Steamer projections, I found the amount of HR per 600 PA change in a player’s debut season. Most seasons, the results and projections differed by only off by +/- 1 HR per 600 PA. In 2016, the major league results were +3.4 HR per 600 PA more than expected. Doing the same analysis with 2017 projections, ZiPS has underestimated MLB results by 4.9 HR per 600 PA while Steamer has underestimated them by 6.5 HR per 600 PA.

I am not going to get into a debate on how and why there are more major league home runs being hit. They just are. For this reason, hitters making the jump from AAA to the majors don’t see their home run numbers drop like in previous seasons. Projection systems have been slow to implement this change and are underestimating major league home runs. When a minor league gets promoted to the bigs, there is no reason to expect a power dropoff and savvy fantasy owners can use this trend to their advantage.