Every team is expected to win more games at home than it does on the road, to the extent that if it only breaks even on the road it is deemed to have a shot at the pennant.
Hitters’ park or pitchers’ park, the home team should take advantage of its peculiarities better than the visiting team.
Why would a team, strong or weak, perform better in their own park than on the road? The players benefit from home stands of reasonable duration(say, eight to thirteen games) when they live their own residences, sleep at more nearly regular times, play before appreciative fans, and benefit from the physical park conditions which to some degree may have made their organizations acquire them in the first place. It is difficult for fans to grasp the difficulty of playing on a travel day or of adjusting to jet lag and hotel «comforts».
Individuals bat and pitch at a rate 10 percent higher at home, on average. That is, On Base Average and slugging percentage each tend to be 5 percent higher (when combined to create OPS, they are 10 percent higher); batting average will be 5 percent higher too. Linear Weights, because it is denominated in runs, will be 10 percent higher at home, while earned run average, for the same reason, will be 10 percent lower. These statements are true on average, but in some cases home park variations may run considerably higher or lower.
Batting statistics reflect not only how players played, but where they played, with the latter proposition having enormous effect on those batters blessed to have played half their games in Shibe Park, Fenway, or Wrigley Field and on those cursed to have been denizens of Yankee Stadium, San Diego Stadium, or the Astrodome.
If we desire to remove the silver spoon or the millstone that a home park can be, and measure individual ability alone, we must create a statistical balancer which diminishes the individual batting marks created in parks like Fenway and augments those created in San Diego.
Parks differ in so many ways that it may be hard to imagine how their differences can be quantified. The most obvious way in which they differ is in their dimensions, from home plate to the outfield walls, and from the base lines to the stands. The older arenas ( Fenway Park, Wrigley Field, Tiger Stadium ) tend to favor hitters in both regards, with reachable fences and little room to pursue a foul pop.
Yet two parks can have nearly equal dimensions( like Pittsburgh’s Three Rivers Stadium and Atanta’s Fulton County Stadium), yet have highly dissimilar impacts upon hitters because of climate(balls travel farther in hot weather), elevation(travel farther above sea level), and playing surface(travel faster and truer on artificial turf). Yet another factor is how well batters think they see the ball. Shea Stadium is notorious as a cause of complaints.
And perhaps more important than any of the objective park characteristics, is the attitude of the players, the way that the park changes their view of how the game must be played in order to win.
The successful team is one that can play its games at home(the game for which the team was constructed) yet is flexible enough to adapt when on the road.
Rather than try to assign a numerical value to each of the six or more variables that might go into establishing an estimator of home park impact, Pete Palmer looked to the single measure in which all these variables are reflected (runs). After all, why would we assign one value to dimensions, another to climate, and so on, except to identify their impact on scoring? If a stadium is a “hitters park”, it stands to reason that more runs would be scored there than in a park perceived as neutral, just as a «pitchers’ park» could be expected to depress scoring.
To measure park impact, Pete looks not at the runs scored by the home team, which may have been put together specifically to take advantage of a park’s peculiar features, but rather those scored by the visiting teams. By totaling the runs allowed at home for all teams in a league year and dividing that figure by the runs allowed by all teams in their road games, we take the first step in determining the Park Factor which may be applied to a team’s batters and pitchers (it might also be applied to base stealers, etc. ).
For most of us, though, it will be enough to understand that the Park Factor consists mainly of the team’s home-road ratio of runs allowed, computed as it was above for the league, compared to the league’s home-road ratio. The batter adjustment factor, or Batter Park Factor (BPF), consists of:
- The Park Factor.
- An adjustment for the fact that the batter does not have to face his own pitchers.
The pitcher adjustment factor, or Pitcher Park Factor(PPF), likewise consists of the Park Factor and an adjustment for the fact that the pitcher does not have to face his own team’s batters. The BPF and PPF are expressed in relation to the average home park factor, which is defined mathematically as 1.00. A park which featured 5 percent more scoring than the average park would have a BPF of 1.05, while that same park’s PPF might be 1.04 or 1.00, for instance, because it is adjusted differently.
A home park with extreme characteristics(heavily favoring pitchers or batters, lefty or right) can be a problem. In Fenway, visiting teams almost never start a lefthander because the Red Sox have historically stacked their lineup with right-handed hitters who can pull 350 feet fly balls over the wall.
Home park characteristics certainly are on the minds of management as they contemplate trades. They may even have been on the minds of Managers.
Park Factor offers a suggestive truth, one that is essentially and logically plausible. But not «verifiable» statistically(statistics never prove, anyway-they are estimations of truth).