How much will Citizens Bank Park help Bryce Harper?

With Bryce Harper signing with the Phillies, there has been a lot of excitement about what a hitter like him could do by moving to the more homerun-friendly confines of Citizens Bank Park.

Fans have been giddily throwing out 40 or even 50 home run estimates for Harper as a Phillie, and we can try to estimate what he might do in a number of ways, some of them easier than others:

1) Career Rates at CBP and elsewhere

Using his career stats since coming into the league:

Adding both together, we get 38.4 for a 162-game season consisting of 81 at CBP and 81 at all other parks.

Maybe his stats as a 19-21 year old aren’t that relevant to what he is today — instead of basing this on his full career, we could include just the last three years, i.e. the years after his historic MVP season in 2015. That would get you essentially the same result, at 38.7 for a full season.

If we wanted to be really optimistic we could use his last four years (including 2015 when he hit 42 total, and five at CBP in just seven games), and that would goose the number up to a Howardian 48.4 per 162.

The Phillies pitching that Harper faced over these last seven years was generally pretty bad, and in fact they had the second highest HR/9 rate in the NL over that period. However that conflates the impact of the pitching and the park. To try to separate those we can also look at their HR/9 in just away games, where they had the 7th lowest HR/9 allowed in the NL, or essentially about average.

2) Overlay CBP on the balls he hit at home in 2018

Another way would be to take his spray chart at home and overlay it on CBP’s dimensions, as this article did recently, using data from baseballsavant.mlb.com:

We took Harper’s 2018 batted balls and overlaid them on a map of CBP. As you can see, all of his 17 home runs hit at Nationals Park last season would also be out of the Phillies’ home park, but there are also nine (possibly 10) balls he hit in D.C. that were doubles or outs, that would be dingers here in Philly.

That would make 26 to 27 homers, just in the home games. Over the last three seasons, only three hitters hit 27 long balls at home (Aaron Judge, Giancarlo Stanton and JD Martinez, all in 2017).

Harper hit 17 at home and 17 on the road last year, so upping his home number to 27 would have made his 2018 total 44 home runs. I think we would all be quite happy if he did that with the Phils.

3) How much does CBP help power hitters?

Yet another way to go about this is a bit more involved: look at how power hitters like Harper have fared at Citizens Bank Park in the past.

The narrative for many years had been that Citizens Bank Park is a “bandbox”. That term isn’t seen much any more, luckily, but it is still often called a “hitter’s park”, when people using the term usually mean home run park, and perhaps kind of assume that if it helps hitters hit home runs, it must help them overall. Or maybe they just don’t think about it that much.

We can look at this by comparing what hitters (of both teams) do in say the Phillies’ home games, with what they do in the same team’s away games.

This mostly controls for differences in rosters, by comparing the same hitters and pitchers, but in two environments: 1) each team’s home park, and 2) the aggregate of all its away parks.

Is CBP Hitter Friendly?

Let’s look at this comparison for Citizens Bank Park since it opened in 2004, first for just home runs, and then for runs scored:

Home runs

Both teams in Phillies’ away games: 2,453 HRs, or 17.1 per 650 PAs
Both teams in Phillies’ home games: 2,898 HRs, or 20.1 per 650 PAs

Over its 15-year life, CBP has increased HRs by 18%. That ranks 5th in MLB over that time.

(Note, many of the stats will be shown on a per-650 PA basis, so that they are like full player season numbers that we’re more accustomed to.)

Runs scored

And the same comparison for run scored, the bottom line for any offense:

Both teams in Phillies’ away games: 10,864 Runs, or 75.6 per 650 PAs
Both teams in Phillies’ home games: 11,089 Runs, or 77.1 per 650 PAs

That’s an increase of 2.0%, which ranks 13th in MLB over those 15 years, or solidly middle of the pack, making CBP one of the more neutral parks in the league for overall hitting and offense.

We can make this comparison for any stat:

Some highlights:

• CBP has increased home runs by 3.1 per 650 PAs, 18% more than the (mostly) same players hit/gave up at the Phillies’ away parks.
• However it has also reduced singles/doubles/triples (i.e. “field hits”) by 4.3 per 650 PAs.
• It has reduced walks somewhat (0.5, or 1%), but increased Ks much more (5.4, or 4%).
• Bottom line: it has added, on average, only 7 points of OPS (1.0%), and 2 points of wOBA (0.7%).

Circling back to the question above:

Is CBP Hitter Friendly? Meh. A 2.0% increase in scoring (13th in MLB), or 2 points (0.7%) of wOBA is not the impact most people have in mind when they say “hitter friendly”.

So it doesn’t help hitters much on average, because while it increases home runs, it also takes away other hits. But that 18% increase in home runs may mean that it helps home run hitters.

Does CBP help home run hitters?

The comparisons above were done at the team level, but we can compare the same thing for individual hitters too:

• For each Phillies hitter, stats at CBP and stats in away games.
• For each opposing hitter, stats when visiting CBP, and stats when at home hosting the Phillies.

All hitters were categorized by their home run rates: 0 to 5 HRs per 650 PAs, 5 to 10 per 650, and so on. For this categorization only stats in away games were used, to avoid any effect of extreme home parks. Players needed a minimum of 500 PAs in away games over this period (2004-18) in order to qualify.

We end up with a pool of 784 players:

53 players: 0-5 HR per 650 road PAs (really 0-4.999...)
128 players: 5-10 HR per 650 road PA
144 players: 10-15 HR per 650 road PA
162 players: 15-20 HR per 650 road PA
146 players: 20-25 HR per 650 road PA
91 players: 25-30 HR per 650 road PA
44 players: 30-35 HR per 650 road PA
16 players: 35+ HR per 650 road PA

By comparing the stats at CBP for each cohort of players to the cohort’s stats at other parks, we get the differences below.

To pick one example for illustration: there were 44 players who averaged 30-35 HRs per 650 PAs in all of their away games.

These 44 players hit a combined 155 HRs in 2975 PAs in the Phillies’ away games (33.87 per 650 PA). The same group of players hit 193 HRs in 2915 PAs in the Phillies’ home games (43.04 per 650 PA). The difference was 9.2 HRs per 650 PA, or 27%.

Highlights:

• Boy those little guys and speedsters really have gone to town at CBP, upping their HR rate a whopping 183%, from 1.7 per 650 PAs, to 4.8.
• Even those large increases in HRs though in the end translate to minimal impact in OPS and wOBA (both up just 1% overall).

That’s a lot to make sense of, so let’s boil it down some into three main groups:

• low power (0 to 15 HRs per 650 PAs): 325 players
• medium power (15-30 HRs per 650): 399 players
• high power (30+ HRs per 650): 60 players

In short, while it’s not a linear relationship necessarily, the more home runs a player hit, the more that CBP helped their overall stats.

Low power guys (0-15 HR) saw an increase in home runs (2.2 per 650 PAs, or 22%), but also lost 6.1 other hits. In the end CBP was not “hitter friendly” to them at all, reducing both their OPS and wOBA very slightly.

Hitters with medium power (15-30 HR) gained 3.0 HRs, but lost 2.2 other hits and ended up with slight increases in both OPS (1.5%) and wOBA (1.3%).

The high-power hitters (30+ HR) also lost singles and doubles, but their higher HR totals more than made up for that, bringing their OPS up 36 points (4.2%), and their wOBA 12 points (3.3%).

So yes, CBP does help power hitters to an extent.

With 30.4 HRs per 650 road PAs so far in his career, this is the group Bryce Harper falls into.

We can also be more specific and see whether the fact that he’s left handed helps more, or less.

Does CBP Favor Lefty Hitters More?

In short, yes.

First, for right-handed batters:

Right handed hitters as a group did hit more home runs, but overall their OPS and wOBA were almost unchanged, though it varied by power level.

With left-handed hitters, we do see some clear positive effect, especially for the 30+ HR guys, with increases of 39 points in OPS, and 13 points in wOBA:

What’s the bottom line for Harper?

Lefty power hitters (30+ HR), on average have seen a 17% bump in home runs, and 4.5% bump in OPS. Applying these increases to his 2018 home stats, would add 3 HRs and about 34 points to his full season’s OPS:

HRs: +3 from 34 to 37
OPS: +34 pts from .889 to .923

But the average is an amalgamation of some players who were helped more, and some who were helped less. Based on his seven years in the league, Harper has been helped more than most so far, and while his 208 PAs isn’t a large sample, it’s big enough that the HR rate may have stabilized.

Citizens Bank Park should help Harper, especially in his home run totals. It may lower his batting average at the same time, but the net effect should still be positive overall.

By the way

As a final note, this comparison of CBP has all been about the 15 years since it opened, and its overall stats over that time: an 18% increase in home runs, but only a 2% increase in scoring, and 1% (or less) increase in OPS and wOBA.

However it was somewhat more hitter friendly early on, and if we limit the view to just the last 10 years (2009-2018), the increase in HRs is nearly the same (16%), but the overall effects of CBP are even less pronounced:

Both teams in Phillies’ away games: 6,968 Runs, or 73.5 per 650 PAs
Both teams in Phillies’ home games: 6,962 Runs, or 73.2 per 650 PAs

That’s a reduction of ¹⁄₂ percent in scoring, to go along with similar reductions in both OPS and wOBA: