Birthday Bashing

– The K Zone –


Birthday Bashing, by Ian Joffe

July 4th, 2018

In honor of America’s 242nd birthday, we’re going to take a look at how players hit on their own birthdays. A strict “clutch hitting does not exist” model would argue that there should be no difference in batting outcomes, but a more psychologically based view could see significant changes in hitters’ approaches and results. It certainly seems as if some players consistently hit on their birthday, but that could be by random chance, or only apply to some. The question is whether a larger trend exists.

To test the two theories against each other, I compiled data from The Lahman Database and Retrosheet (two excellent sites and sources, by the way) using python. In total, there were 478 times in 2017 in which a player made a plate appearance on their birthday, which is not a huge sample size, but it should be enough for most stats. We will turn those 478 PA’s into a conglomerate player, who we’ll call the “birthday boy.” Here’s a comparison between The Birthday Boy’s numbers and the league averages from the 2017 season:

  League Average Birthday Boy
AVG .255 .263
HR% 3.3% 3.6%
K% 21.6% 22.2%
BB% 8.5% 9.4%

At least last season, there was no indication that players did better on their birthdays than any other day. The small differences in, say, BB% do not come close to holding significance when a t-test is applied. Not only do players not improve in power on contact by a statistically significant amount, but it seems their approach does not change either. Players are not more anxious and aggressive, nor are they more nervous and passive, according to strikeout and walk rates.

All in all, it looks like the robot within us has this one locked up. Don’t go picking up players in DFS just because it’s their birthday, and don’t count on your 25th man to come up clutch just because it’s his special day. There will be plenty of opportunities for clutch hitting down the stretch, when the team needs it most.



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