The Z Files: Looking Back at 2018 Park Factors

One of the first things I do when the season ends is run park factors. Part of it is practical, as they're needed for 2019 projections, but they're also fun to look at. Other than checking out the parks at the extremes, several venues piqued my curiosity. Front and center is Chase Field and the effect of the humidor. Another is SunTrust Park, as we now have a second season of data. Similarly, this is the second year Minute Maid Park played without Tal's Hill. Finally, one of the under-the-radar changes was Angel Stadium lowering the home run line in right field from 18 down to eight feet.

Before reviewing the 2018 factors, keep in mind applied park factors are three-year averages to flesh out the residual variance. The algorithm is designed to minimize the influence of the team by comparing hitting and pitching results home and away. That is, a hitter-friendly park isn't a result of a strong offense or weak staff, while a pitcher-friendly venue isn't due to a solid rotation or punchless lineup. Further, a venue favorable for homers isn't necessarily advantageous for runs, while a power-suppressing venue can be a boon for runs if it means more balls finding outfield grass. For a more comprehensive description of park factors, check out a piece I wrote in April 2017, The Necessary Evil of Park Factors.

Another important thing to understand is that park factors do not apply to all players in a linear fashion. Plus, when the data is distilled to a single player, the noise surrounding the outcomes could easily mask any tangible influence exerted to the player.

A couple of interesting variables this season are the inclement start to 2018 along with the proliferation of matinee baseball. The former is an aberration. However, the 2019 schedule appears to have a similar number of day games, working in tandem with increased off days to make traveling less burdensome. The sample of any one team playing fewer games under the lights isn't going to be enough to make a difference, but league-wide it could be telling, though one year's worth of data isn't enough to do anything more than keep an eye on things and maybe act on it as a tiebreaker. I'll be looking at the difference in run-scoring between day and night games later in the offseason.

Now let's look at some of the specific venues.

Chase Field

Speaking of archives pieces, in the spring I wrote a column for ESPN, How the humidor will impact the Diamondbacks' hitters and pitchers. Prior to the installation of a humidor, Chase Field inflated runs significantly. The party line was that the humidor was deployed so the game balls would be less slick, and easier to grip for pitchers. However, the added moisture also serves to decrease the elasticity of the ball, reducing exit velocity. Thus, homers were expected to fall, and they did with a 94 homer index as compared to a 2015-2017 mark of 109. This is only one season of data so the jury is still out, but it did head in the expected direction.

In the above piece, I pegged the projected run index to a range from 95 to 105. Granted, that was a large spread, but the error associated with the data dictated the wide range of outcomes. The associated three-year run index was 116. At season's end, Chase Field played to a 105 index. Even though this is within my predicted range, please don't send me YooHoo to pour over my head in a victory celebration. Again, we're only looking at one season. Though I am confident the park will continue to lower scoring, it's the extent that's still up in the air. Not to mention, Chase Field is one of the more variable parks in terms of run index, playing 120, 122, 106, 116 and 97 from 2018 to 2014. In 2016, the park played nearly identically to this season without the humidor, while in 2014, the venue aided pitchers, again with the slicker, bouncier ball.

Even though it's only one season of data, I'm using 2018's run factor of 105 in projections, along with a hone run factor of 94 from both the left and right side. The three-year factor for righty homers was 111 compared to 106 for lefty swingers.

Perhaps the player expected to be affected the most was Paul Goldschmidt, falling from a perennial top-five overall player to someone drafted five to ten picks later. Recall he got off to a maddeningly slow start which many attributed to the humidor. Well, 33 homers later it appears Goldschmidt overcame the mental hurdle of hitting in Chase Field. Or, did he? Goldschmidt swatted 21 homers on the road compared to just 12 in the desert. In the previous two seasons, 35 of his 50 long balls came at home.

As mentioned, one year's worth of data is insufficient to draw any conclusions. The likeliest outcome is more Chase Field dingers with fewer of the road variety next season, but we'll see.

To demonstrate how factors don't affect everyone the same, A.J. Pollock hit 11 of his 21 big flies at home. Ketel Marte muscled up eight times in Chase Field, two more than away from home.

SunTrust Park

In its inaugural campaign, SunTrust Park played a tick below neutral for runs, checking in with a 98 factor. In its follow-up season, this rose to 112, on a par with Fenway Park, one of the better venues for runs in the league.

In 2017, the lefty home run factor was a generous 116, falling precipitously to 73 in 2018. Because the sample of left-handed batters is less than their right-handed brethren, lefty indices tend to be more variable. To that end, the RH home run index was 81 two years ago, rising a little to 87 this season.

The batter most influenced by the 2018 numbers is Freddie Freeman. The first baseman still had a solid season, though more was anticipated in the power department, especially after the park played so favorable for lefties in its initial season. Some may connect the two, figuring Freeman's down season dragged the homer index down, but one player isn't enough to make that much of a difference. If Freeman had smashed 10 more homers at SunTrust without knocking any additional ones on the road, the index climbs to 84, still an extreme power suppressor.

The impending 2019 season will yield the desired third data point. Until then, all we can do is average the first two seasons for SunTrust park and keep in mind the expected production has a wider range than that emanating from more established venues.

Minute Maid Park

Minute Maid Park is one of my favorite venues in terms of factors, as many don't appreciate how much of a pitcher's paradise it has been. This is one of the reasons I was very high on Gerrit Cole entering 2018. As alluded to earlier, Tal's Hill was removed from left-center field prior to the 2017 season. For those unaware, Tal's Hill was literally a hill in deep left center, with a maximum distance of 436 feet from home plate. It had a signature flag pole which was in play. Outfielders were not fond of navigating the incline and risk smashing into the pole. After its elimination, the new fence was 409 feet away, availing the possibility of more homers.

Prior to 2017, the three-year runs factor was 91, a decent pitching venue. Last season, the first without Tal's Hill, the park played an even more stingy 83 for runs. This past year, this spiked to 99. The average of the two post-Tal's Hill seasons is 91, exactly the index prior to the renovations.

With respect to homers, with Tal's Hill, Minute Maid played just under neutral at 99 for right-handed batters. In 2017 it went up to 109 while sitting at 107 this past season. The 2019 results will be telling, as there's a chance Minute Maid will indeed play closer to neutral now.

Angels Stadium

Coming into the year, the three-year park factor for lefty power was 91. With the lowered fence height (the distance remained the same), the factor for lefty homers skyrocketed to 134. Keeping in mind the usual caveat of one year isn't ample to draw concrete conclusions, this bodes well for a certain Tommy John recoveree, as Shohei Ohtani is expected to spend 2019 as the Halos primary designated hitter.