This article is part of our The Z Files series.
On paper, this is a great idea. But sometimes great ideas aren't so great after all. How actionable are in-season park factors? At what point of the season, if at all, are they reliable? Perhaps this assumed edge isn't really an advantage and is in fact hurting, not helping.
Before we embark on that study, below is a chart with the 2013-2015 three-year index for runs, left-handed homers and right-handed homers along with the 2016 index through games played on Wednesday, July 20.
3-Yr R | 2016 R | 3-Yr HR LHB | 2016 HR LHB | 3-Yr HR RHB | 2016 HR RHB | |
ARI | 106 | 119 | 109 | 134 | 94 | 126 |
ATL | 94 | 108 | 86 | 84 | 95 | 67 |
BAL | 107 | 101 | 137 | 83 | 108 | 109 |
BOS | 107 | 106 | 70 | 75 | 100 | 104 |
CHC | 102 | 80 | 97 | 66 | 124 | 98 |
CHW | 98 | 101 | 109 | 146 | 113 | 122 |
CIN | 102 | 93 | 131 | 111 | 117 | 124 |
CLE | 103 | 122 | 105 | 109 | 103 | 109 |
COL | 140 | 162 | 115 | 154 | 121 | 125 |
DET | 101 | 100 | 95 | 149 | 95 | 108 |
HOU | 100 | 86 | 122 | 77 | 112 | 95 |
KC | 104 |
On paper, this is a great idea. But sometimes great ideas aren't so great after all. How actionable are in-season park factors? At what point of the season, if at all, are they reliable? Perhaps this assumed edge isn't really an advantage and is in fact hurting, not helping.
Before we embark on that study, below is a chart with the 2013-2015 three-year index for runs, left-handed homers and right-handed homers along with the 2016 index through games played on Wednesday, July 20.
3-Yr R | 2016 R | 3-Yr HR LHB | 2016 HR LHB | 3-Yr HR RHB | 2016 HR RHB | |
ARI | 106 | 119 | 109 | 134 | 94 | 126 |
ATL | 94 | 108 | 86 | 84 | 95 | 67 |
BAL | 107 | 101 | 137 | 83 | 108 | 109 |
BOS | 107 | 106 | 70 | 75 | 100 | 104 |
CHC | 102 | 80 | 97 | 66 | 124 | 98 |
CHW | 98 | 101 | 109 | 146 | 113 | 122 |
CIN | 102 | 93 | 131 | 111 | 117 | 124 |
CLE | 103 | 122 | 105 | 109 | 103 | 109 |
COL | 140 | 162 | 115 | 154 | 121 | 125 |
DET | 101 | 100 | 95 | 149 | 95 | 108 |
HOU | 100 | 86 | 122 | 77 | 112 | 95 |
KC | 104 | 123 | 88 | 95 | 82 | 80 |
LAA | 92 | 99 | 85 | 109 | 90 | 109 |
LAD | 90 | 78 | 115 | 138 | 101 | 83 |
MIA | 100 | 88 | 64 | 128 | 78 | 75 |
MIL | 107 | 103 | 133 | 134 | 123 | 130 |
MIN | 104 | 105 | 90 | 77 | 96 | 102 |
NYM | 87 | 104 | 124 | 105 | 82 | 138 |
NYY | 102 | 99 | 129 | 135 | 126 | 158 |
OAK | 95 | 86 | 75 | 85 | 90 | 74 |
PHI | 102 | 76 | 110 | 124 | 140 | 95 |
PIT | 94 | 106 | 92 | 119 | 73 | 65 |
SD | 93 | 104 | 110 | 70 | 110 | 108 |
SEA | 90 | 93 | 98 | 116 | 91 | 108 |
SF | 88 | 94 | 59 | 41 | 76 | 74 |
STL | 97 | 86 | 87 | 88 | 90 | 82 |
TB | 96 | 83 | 109 | 85 | 83 | 84 |
TEX | 106 | 114 | 101 | 99 | 95 | 106 |
TOR | 102 | 104 | 114 | 109 | 121 | 8 |
WAS | 103 | 94 | 80 | 82 | 88 | 91 |
Let's break things down in terms of the percent change of 2016 from the three-year averages.
There are seven venues playing at least 10 percent more pitcher friendly this season: Citizens Bank Park, Wrigley Field, Minute Maid Park, Tropicana Field, Dodger Stadium, Marlins Park and Busch Stadium. The initial pair are playing over 20 percent more pitcher friendly.
There are eight venues playing at least 10 percent more hitter friendly: Petco Park, Chase Field, PNC Park, Turner Field, Coors Field, Kauffmann Stadium, Progressive Field and Citi Field, the last of which is exactly 20 percent different.
Eight venues are 10 percent or more less homer friendly for righty swingers: PNC Park, Minute Maid Park, O.Co Coliseum, Dodger Stadium, Wrigley Field, Rogers Centre, Turner Field and Citizens Bank Park. The last four are at least 20 percent worse.
Seven venues are at least 10 percent more home run friendly for right-handed batters: Citi Field, Chase Field, Yankee Stadium, Angels Stadium, Safeco Park, Comerica Park and Globe Life Park. The first four are at least 20 percent better.
Nine venues are at least 10 percent less home run friendly for left-handed hitters with the last six over 20 percent: Target Field, Great American Ballpark, Citi Field, Tropicana Field, AT&T Park, Wrigley Field, Petco Park, Minute Maid Park and Camden Yards.
Eleven venues have increased left-handed power by at least 10 percent, the first eight checking in at least 20 percent: Marlins Park, Comerica Park, U.S. Cellular Field, Coors Field, PNC Park, Angels Stadium, Chase Field, Dodgers Stadium, Safeco Field, O.Co Coliseum and Citizens Bank Park.
Let's summarize as there's something very telling in this data:
Runs | RHB HR | LHB HR | |
10 pct plus higher | 7 | 7 | 11 |
20 pct plus higher | 1 | 4 | 8 |
10 pct plus lower | 8 | 8 | 9 |
20 pct plus lower | 2 | 4 | 6 |
Note how the variance follows the size of each sample. Runs has the largest sample since it's not parsed into handedness with RHB HR having a larger sample than LHB HR. Runs and RHB HR have the same number of parks that are 10 percent different but there are more RHB HR over 20 percent. Runs and RHB HR each have half the total parks with a 10 percent difference while the LHB HR index shows a difference for 20 venues, the majority with factors differing by more than 20 percent. This suggests the variance could be sample-size related rather than a true reflection of any 2016 changes.
It's interesting to note the huge difference in Marlins Park, especially for left-handed power. Remember, they lowered and moved the right-center field fences in. Spoiler alert: while some of the jump to 128 from 64 is likely real, the sample is too small to comfortably call The Aquarium (that really should be the park's nickname) a very favorable venue for lefty pop.
OK, let's get into the meat of this discussion and address the question of in-season sample size reliability. I took each team's game logs from 2013-2015 and dumped them into a program that calculates first half and second half park indices along with a control set. Actually, the distinction isn't exactly first half and second half but first 41 games played in each venue versus final 40. The control set measured the park index of the odd numbered games and the even numbered, yielding the same 41 to 40 comparison.
The actual data goes beyond the scope of this discussion but the results were clear. The correlation between the first half/second half data to three-year average was the same as that of the odd/even data – essentially nothing. In other words, identifying the outlier parks above was mostly an exercise in futility.
Or was it? Knowing the factors others are using is always helpful, especially in DFS if you're looking for a contrarian angle. Let's circle back to the LHB HR index in Marlins Park. At some point, people are going to discover, if they haven't already, how much the index has increased. There really aren't many left-handed Fish hitters to take advantage (Christian Yelich, Derek Dietrich, Justin Bour when he returns) but I'll wager someone will highlight an opposing lefty swinger as a strong play based on the huge upgrade in lefty power. I'm fading that hitter unless the spot is too strong to ignore, like Bryce Harper facing Jose Urena. It's more not taking the bait on an average lefty – Cody Asche, for example. It's really easy to envision a scenario where Asche is on someone's value play list because Marlins Park is perceived to be more generous to lefties.
This study begs a follow-up which I'm in the process of undertaking. How many games are needed to show correlation? The plan is to keep increasing the sample by ten games to find the spot where correlation exists, supported by a control set of like size. I'll use the individual game logs from a full season and compare the respective factors after 90, 100, 110, 120 games etc, in the hopes of finding when the in-season index is actionable. Of course, we're still working blind until then, but at minimum we'll be ahead of the game for the most important part of the season: DFS championship tournament time and down the stretch in seasonal formats.
For what it's worth, this code can be tweaked to do a similar study on the in-season reliability of weighted on base average, strikeout percent or any of the other metrics commonly employed for in-season analysis.