Park Factors

Baseball is the only major sport where the field itself differs from venue to venue — fence distances, wall heights, foul territory, altitude, and the air the ball travels through. A ball hit 380 feet to left-center can be a home run in one park and an out in another.

A park factor expresses this as a multiplier. A home run park factor of 1.15 means roughly 15% more home runs are hit there than in a neutral park, holding the quality of players roughly constant.

Park factors are computed by comparing what happens in a team's home games to what the same teams do on the road, which cancels out most of the difference in team quality.

They are, however, blunt:

• They are noisy year to year. Single-season factors bounce around; multi-year versions are steadier.
• They are not one number. A park can suppress home runs and inflate doubles at the same time, and can play very differently for left-handed and right-handed hitters.
• They do not capture weather on the night in question. A park factor is an average across conditions.

Use them to adjust your expectations, not to compute an exact answer.

When a hitter changes teams and his numbers move, the first question is whether he changed or his environment did. A hitter moving from a pitcher-friendly park to a hitter-friendly one will usually post better raw numbers without having improved at all.

This is why park-adjusted metrics exist. When you see a stat labelled "plus" or "adjusted" — OPS+, wRC+, ERA+ — the adjustment for park is generally already inside it, with 100 set as league average.