Sabermetrics: Park Factors

By Wikipedia

Batting Park Factor, also simply called Park Factor or BPF, is a baseball statistic that indicates the difference between runs scored in a team's home and road games. Most commonly used as a metric in the sabermetric community, it has found more general usage in recent years. It is helpful in assessing how much a specific ballpark contributes to the offensive production of a team or player.

The formula most commonly used is:

In this formula, all runs scored (by or against) a team at home are divided by all runs scored on the road. Parks with a Park Factor over 1 are those where more overall runs are scored when the team is at home than are scored when the team is away. While some variation in the can be attributable to fluctuations in offensive and defensive performance, PF accounts for the production of both teams in each park and, correspondingly, is very useful in determining which actual ballparks are "hitter friendly" and which are "pitcher friendly".

In place of Runs scored and allowed, the formula can easily use home runs, hits or any other statistic to further analyze the park factor of an individual park.

On our yearly team pages (such as the 1999 New York Yankees), we list the figures for both Batting Park Factor (BPF) and Pitching Park Factor (PPF). It is assumed that a BPF over 100 means a batter benefited from playing in this park while a PPF under 100 means a pitcher benefited from pitching in this park.

A more detailed formula that shows how these 1-year averages were arrived at is located about halfway down this page under the "Park Factors" heading.