### Beat the Streak: Day Seven

In this blog post, I discuss questions of the form: “Does batter X perform better at home or away?At day or night?Against lefties or righties? On Friday or Monday?”What I found was a little bit surprising.

Take for example the batter Daniel Murphy.When you look at his data from 2011 - 2017, you will see that he got a hit in 29.85% of 1424 plate appearances during day games and he got a hit in 26.97% of 2673 plate appearances during night games.This is a pretty meaningful difference, but is it statistically significant?In other words, could this difference be explained purely by chance?To answer this question, we can perform a chi squared test under the null hypothesis that the true probabilities are the same.When we do this we get a chi squared value of 3.35 and a corresponding p value of 0.067.Thus, we can reject the null hypothesis that the true underlying probabilities are the same at the 90% confidence level.This is pretty convincing evidence that the day/night split really matt…

Take for example the batter Daniel Murphy.When you look at his data from 2011 - 2017, you will see that he got a hit in 29.85% of 1424 plate appearances during day games and he got a hit in 26.97% of 2673 plate appearances during night games.This is a pretty meaningful difference, but is it statistically significant?In other words, could this difference be explained purely by chance?To answer this question, we can perform a chi squared test under the null hypothesis that the true probabilities are the same.When we do this we get a chi squared value of 3.35 and a corresponding p value of 0.067.Thus, we can reject the null hypothesis that the true underlying probabilities are the same at the 90% confidence level.This is pretty convincing evidence that the day/night split really matt…