How Bigger Bullpens Are Constraining Offense

This is a story about persistence. I thought I had an interesting way of looking at the marked decline in scoring this year so far; as it turns out, I was wrong. A further investigation, however, revealed that another possible culprit was right around the corner. Does it explain the entire decline in offense? Most certainly not. But I’m interested nonetheless, and I hope you will be too.
If you’ve followed baseball in the past five years, you’re probably used to asking questions about league-wide offense early in the season. Major League Baseball has done itself no favors here; the composition of the baseball keeps changing, and home run rates fluctuate wildly as a result. The same is true this year: despite the adoption of a universal DH, offense is down across the board.
The usual suspects are certainly part of the problem. Pitchers keep throwing harder. Putting a humidor in every stadium affects home run rates in unpredictable ways and might suppress home run rates early in the season. The league used two different baseballs last year, and drag coefficient is up this year. Starters are going fewer and fewer innings, giving batters fewer looks at them a third time through the order.
I think that all of those things have something to do with bad offense. But I thought of another potential cause, one I could investigate without learning fluid dynamics. One of the side bargains between the league and the MLBPA after this offseason’s lockout was for expanded rosters early in the season. Teams are allowed to roster 28 players throughout the month of April. On May 30, that number will revert to the standard 26 — this deadline was recently pushed back from May 1. In addition, teams can carry any number of pitchers on their roster until May 1. After that, they’ll be limited to 14, and 13 after May 29.
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