With the addition of Wei-Yin Chen, many Fish fans have started to take a closer look at his scouting report. As has already been covered, Chen is pegged as a fly-ball pitcher whose biggest concern is his susceptibility to giving up the home run. When we dig deeper into park factor and Chen's statistics, however, we see that this fear is greatly overblown.
An interesting way to analyze a pitcher's home run susceptibility is by looking at HR/FB. This is a sabermetric statistic that looks at a pitcher's home runs allowed divided by his fly balls allowed. It is frequently presented as a percentage. Here is the formula written out:
So what does this tell us? It gives us an idea of how "real" a pitcher's raw numbers are. For example, if a pitcher's HR/FB is incredibly high compared to league average, we can assume that he has been "unlucky" or perhaps pitches in a "hitter friendly park." On the contrary, if a pitcher's HR/FB is league average yet he is still giving up a lot of home runs, this is an issue. It signifies that he is susceptible to the home run ball.
As our Michael Jong has already written, Wei-Yin Chen is about to go from pitching in an EXTREME hitters park to an EXTREME pitchers park. Want a refresher? Take a look at this table:
I took the number of home runs hit from 2013-2015 in each major league park and summed them to rank stadium susceptibility to the long ball. In that three-year span, Camden Yards has seen 263 more home runs than Marlins Park. That is a staggering number. Camden Yards is the fourth most susceptible park to the long ball. Due to this, you would also expect Chen's HR/FB to be higher than average, and it is.
In 2015 the league average HR/FB percentage was 10.1 percent, meaning that on average, 10.1 percent of all fly-balls hit in the MLB went for home runs. In 2015, Chen's HR/FB percentage was 12.3 percent. That number is high and suggests there is more than meets the eye to Chen's home run problems.
When we look back at Camden Yard's susceptibility to home runs, however, Chen's high HR/FB percentage makes sense. In fact, the HR/FB percentage across the entire Orioles pitching roster was 11.7 percent, second in the league. The Marlins? About nine percent at 9.2 percent, second lowest in the league. Those statistics are absolutely a product of the home ballparks. Yet, Chen's HR/FB percentage is even higher than that of the Orioles' staff, indicating that he might have also been "unlucky." Such a high HR/FB percentage is usually considered unsustainable and it is expected that a pitcher will regress back toward the mean.
Before anyone worries about Chen's home run susceptibility, let's wait and see how he fairs in a park that has surrendered the fewest home runs in the league since 2013. While still a fly ball pitcher, Chen has also begun to induce more ground balls. From 2012-2015 the percentage of ground balls hit against him in play has increased from 37.1 percent to 40.5 percent. This, and the effect of moving from Camden Yards to Marlins Park, should lead to an improvement in Chen's "home run problem," if it exists at all.
HR/FB statistics were taken from Fangraphs.