First Inning Home Field Advantage

The home team has consistently, on a year-to-year basis, won 54% of its games. Several reasons have been explored for the disparity, such as familiarity to the home field and the umpire’s biased strike zone. Another aspect that comes into play is a first-inning discrepancy in favor of the home teams’ pitchers. They have an abnormally large advantage in strikeout and walk rates, partially because of a higher fastball velocity.

Note: For consistency throughout the article, when I refer to K/BB, it will be in reference to pitchers.

With better use of bullpens and more patient hitters, strikeout and walk rates have climbed in recent years. Since 1950 (the extent of Retrosheet’s data set), the home team has always maintained a higher K/BB ratio than the away team.

While the strikeout portion of K/BB has outpaced the walk portion, the ratio of home K/BB to away K/BB has remained fairly constant — averaging 1.14 and bouncing between 1.09 and 1.19. While trying to remove some of the noise from the data, I figured I would remove the ninth inning, when the away pitcher doesn’t always pitch. While looking at inning-by-inning data, I found the first inning exhibited the greatest K/BB ratio, compared to other innings.

Usually, the first inning is a little rougher for pitchers since they are guaranteed to face the top of the other team’s lineup. The second inning sees the highest K/BB values since the pitcher usually faces the bottom part of the lineup. After the second inning, the order become fairly random.

The ratio was at 1.3 for the first inning, then declined as the game went went on to a low of 1.07 for the ninth inning. I know some people prefer K% and BB%. The K%-BB% for the home team was 6.8% in the first inning and 3.6% for the away team (3.2% total difference). By the ninth inning, the total difference was .5%

While reading many articles on this subject I came across this little tidbit, from our own Matt Swartz, while he was at Baseball Prospectus:

That first-inning differential is particularly interesting, because it could be caused by another factor. Instead of the issue being that the home pitcher was more familiar with the mound than the visiting pitcher, perhaps the home pitcher is just more prone to be more effective immediately after warming up. Maybe a home pitcher knows the exact moment that he is going to step on the mound and can warm up until a couple minutes beforehand, and maybe a visiting pitcher loses focus and cools off while in the dugout waiting for his team to finish batting in the top of the first inning before getting the ball. While first warning readers about the small sample size, I can now say that we do see some clear indications that the first-inning warm-up effect may be part of home-field advantage.

Could it be the away pitcher just sits too long? Normally, the starting pitchers join their teams for the “The Star-Spangled Banner.” Then, the home starter takes the mound. The away team’s starter heads to the dugout for his warmup jacket. Here are the average per inning fastball and sinker speeds, since 2007.

Note: I only looked at the first five innings in order to minimize the effect of relievers entering the game.

In the first inning, home starters averaged a fastball .15 mph faster than their away counterparts. The difference increased to .19 mph if the away pitcher’s team scored any runs thereby extending the time he was on the bench. Here is a chart on how fastball speed declined as the starter waited longer for his team to finally quit hitting.

The decline became significantly pronounced once the away team made the starting pitcher wait five minutes or more for runs to be scored.

So the home starter throws harder to begin with — but does this difference in velocity really matter? The short answer is no. It makes a nice difference in the first inning, but as the game goes on, the away team actually sees a very slightly higher velocity. Going back to the original question of home field advantage — and using some of Mike Fast’s previous work — higher velocity ends up explaining one run of the home field advantage over an entire season.

While the difference is not huge at the league level, it’s more pronounced at the pitcher level. Here are the 11 pitchers who have seen a one mile-per-hour-or-more drop in velocity between throwing in the top of first inning compared to the bottom of the inning (2013 data, minimum 1,000 fastballs thrown)

Name	Top of the 1st	Bottom of the 1st	Difference
Hyun-Jin Ryu	91.0	89.4	1.6
Corey Kluber	94.4	92.9	1.5
Mark Buehrle	83.8	82.4	1.4
Joe Blanton	89.7	88.3	1.4
John Danks	89.3	88.1	1.2
Kyle Kendrick	89.6	88.4	1.2
Julio Teheran	92.4	91.2	1.2
Dan Haren	87.5	86.4	1.1
Yu Darvish	92.4	91.4	1.1
Jered Weaver	87.8	86.8	1.0
Ervin Santana	93.0	91.9	1.0

Some home teams should allow certain starters to go back to the bullpen to finish their pregame warm ups while his teammates are batting. This may cause a little problem in the National League — where the pitcher might end up batting — but if a pitcher is hitting in the first inning, the inning has been a good one. I think the team could plan on using the time the home starter warms up to start the inning and set some time to account for the first three batters. Once this second warm-up is complete, he could then venture to the dugout.

Over the years, the home team has an advantage in the first inning when it comes to the team’s pitchers throwing more strikeouts and allowing fewer walks. Part of the advantage can be attributed to the home pitcher having less time to cool off. The longer the pitcher sits, the slower they seem to throw when they finally take the mound. While the velocity loss from sitting in the dugout does not explain all the difference in strikeout and walk rates, it is a factor. Additionally, the drop off is larger for some pitchers than for others.