### Home Team Strike Bias

In my last post, I looked at the historical pattern of strikes. Since the rate of striking out has been increasing in recent years, that motivated looking at the pattern of the rates of swinging strikes, called strikes. But actually my original interest in this problem was not on the historical patterns of strikes, but rather to explore the **home team bias** in rates of strikeouts and in strikes.

In one of the past blog posts, Brian Mills explored the location of called strikes and demonstrated that umpires have different strike zones for home and visiting hitters. (Sorry, I wasn’t able to find this post, but I know Brian has explored this.) Here I will try to confirm Brian’s conclusion by looking at strike data for the 2016 season.

### Strikeouts

First, I looked at strikeouts and looked at the K rates for home and visitors for each team in the 2016 season. The plot below graphs the bias K Rate(Visitor) – K Rate (Home) against the overall K rate for all 30 teams. Note that most teams fall above the line at zero, indicating that visitors are more likely to strike out.

### Moving On to Strike Rates

This scatterplot of K rates and strike rates was made just to convince us that the two variables are positively associated. The fitted line tells us that a strike rate of 45% will predict a K rate of 22%.

### Two Type of Strikes

I’d thought it would be interesting to look a little deeper and graph the called strike rate against the swinging strike rates for all teams. Although there does not appear to be a relationship between the two rates, some interesting values pop up. Houston has an unusually high swinging strike rate, followed by Tampa Bay and Philly. With respect to called strike rates, Colorado is unusually low and Miami is unusually high. What does this mean? Does Houston have a high swinging strike rate since the batter visibility is not as good there and so hitters tend to miss pitches? Since curve balls don’t curve in Colorado, is that the reason for a low called strike rates? I’m just offering some suggestions. It would be useful to see if these patterns are consistent for recent seasons.

### Home Advantage

Let me offer one way of quantifying the home advantage in strikes. First consider the called strikes — we compute the Called Strike Rate (V) – Called Strike Rate (H). Since we believe there is a home advantage, we would anticipate this difference to be positive. Below I plot this difference for all teams — the red line indicates 0 and the blue line is the average difference (0.0036) across all teams. For a majority of the teams, the visitors are more likely to get a called strike.

Here I repeat this graph for swinging strikes. Here the average difference (across all 30 teams) is 0.0029. In other words, the visitor’s swinging strike rate is 0.29 % higher than the home’s swinging strike rate.

### Comments

- Although these differences in strike rates appear small, they really are a big deal since they translate to fewer runs scored.
- Are these umpire effects? It would seem that the umpires would have the most impact on called balls and strikes and less impact on swinging strikes. But the called strikes might cause the hitter to swing at less-than-desirable pitches, so there is an indirect effect of the umpire on swinging strikes.
- My definition of a home bias in strike rates is faulty since the impact of the umpires and the strikeout tendencies are confounded. Maybe a particular team has a large away minus home difference since that team generally does not strike out. I think this effect is real here since most teams have positive away minus home effects. But to measure this effect for a single team one would need to make a suitable adjustment for the home team’s strikeout ability. (I’ll leave the adjustment to the curious reader.)
- It would be interesting to do a study that examines carefully all of the measurements in a baseball game that are impacted by the home/away bias. We know that home teams are more likely to win than visiting teams, and this study would help to explain why.
- For further reading, here is an interesting study from the Hardball Times on the same issue.