The dreaded ‘hometown scoring bias’ can go in either direction depending on the situation. Your pitcher is in the midst of a no-hitter? That’s an error. Your hitter is in the midst of a hit streak? That’s a hit. These sorts of situations make it tough to see a pattern through all the noise.
That didn’t stop Doug at HighHeatStats. By taking away errors from home errors for both the home team and away team at each park, he tried to find systematic pro- or anti-error bias in each stadium’s scoring team. It’s a little hard to read the graph, but here’s what he found:
If you look at a team like the Angels or Dodgers, you’ll see an interesting phenomenon. The home team sees more errors on the home, and the away team sees more errors in Los Angeles. That sort of split indicates that the Angels, Dodgers, Marlins and Cubs scoring teams may have a bit of home-town bias and prefer to hand errors to the home team while keeping the opponent team’s sheet clean. It’s probably not a front-brain thing — you watch a team for long enough and all it takes is a slight positive feeling towards a pitcher to say, yah, that fielder shoulda gotten to that one, it was a squibber, not a real hit.
The first reaction to this piece might be a general one: errors are silly. Strange that this determination, of what ‘should’ have been a hit, is so pervasive and important to so many of our statistics.
The second might be: lucky pitchers. Because a tendency to call a hit a hit could balloon an ERA. Here’s a case where home-town bias actually helps one side of the team (better ERA!) while hurting the other side (sorry, hitters).
But so far this is just an intuitive response. Is there evidence that these four scoring teams have impacted the stats of the players in their parks? If there were, we’d have our roto-relevancy.
Over the last three years, the Angels and Dodgers both had better home ERAs then FIPs. That might be indicative of a scoring bias problem, except that the Marlins and Cubs have both had home ERAs that are higher than their FIPs. And FIP only credits actual home-run rate, so those two homer-suppressing parks had a little help in outperforming their FIP. Using xFIP doesn’t help, either, as the Dodgers and Angels still sit on one side, and the Marlins and Cubs on the other.
Perhaps the problem is that these biases don’t hold from year to year. At the end of his piece, the author tries to look at the four parks over time, and the only team that consistently showed a bias was Florida/Miami. If you focus just on the Marlins, you’ll see that they have the third-largest difference between home ERA and FIP over the last three years… except in the wrong direction. Their team ERA has been .28 higher than their team FIP, even though errors have been ‘helping’ their ERA.
Not all research leads to clean takeaways. Perhaps the easiest takeaway here is one that calls back to last week’s findings: keep it simple, stupid. For pitchers, focus on strikeouts and walks, because once it’s in play, it’s impossible to know what should be a hit. Even if you try to correct for bias among the scorers.