Home Field Kingdome v Safeco

Jeff has already mentioned how the park factors between the Kingdome and Safeco Field can have dramatic impacts. And showed that the Mariners experience one of the bigger differences in winning from playing in Safeco compared to other teams and their respective home parks.

That is not to say exactly that the Safeco is beneficial to the Mariners. We know the team has raked in enough cash to put the Snoqualmie and Tulalip casinos to shame* with the extra revenue Safeco generates. That money helps to make the team better so in that sense, there is no doubt that the team is better off in their new digs. 

*WE COME AGAIN!

The fans seem to enjoy it more as well even if it is a most costly experience now. However, what interested me was the relative differences between the two stadia. Looking at the difference between home and road winning percentages obscures where the gap comes from. The Mariners could be taking advantage of Safeco or the parks quirks could be coercing them into building a team that hurts them on the road. Figuring out which would take all kinds of normalizations for how we expect the team to perform in various environments and then comparing that to the actual results, which would then be fraught with small samples. In short, it's a difficult task to pinpoint the effect.

The results are easy enough to measure though and thinking about the previous two posts piqued my interest in seeing how the Mariners' difference in winning percentage has changed over time. I extended what Jeff did with winning percentages and went through each season, charting the Mariners home and away records and graphing the difference in winning percentages. The black and blue lines represent the respective averages for each park.

It is a small difference --a 7% average in the Kingdome up to 9% in Safeco-- but the Mariners have seen a wider gap since moving across the street.

You might notice that 2001 was one of just four seasons where the Mariners had a worse record at home than on the road. They were 59-22 away from Safeco that year and "just" 57-24 inside it. The biggest gap was the shortened 1981 season where they were a mediocre 24-28 on the road but a repulsive 20-37 at home.

Interesting to me is how much more stable the gap has been since Bavasi took over. While the difference jumped around a lot nearly every single year, it has been between 11% and 15% since 2004. That could just be a statistical blip or (if I wanted to hypothesize) it could be the math of park factors gaining more widespread acceptance and our front office (even Bavasi's) figuring out better how to cater to the constraints.

Or maybe it was all because of Jose Lopez and the team is headed for some wildly skewed 2011 season.