Gentile: How has reliever usage changed over past 50 years?

From James Gentile at Beyond the Box Score on December 4, 2012:

Beyond The Box Score’s staff Ace Glenn DuPaul has been doing some very interesting work regarding the differences between starters and relievers over at THT the past few weeks. In the first article of the seires, Glenn asked if relievers across baseball were consistently improving, citing an ever-narrowing RA9 gap between RP and SP as evidence.

Commenters at THT and at the Book Blog all had numerous theories to pose as to why this was true. In particular, I noticed two primary hypotheses recurring among them:

1.) More same-handed match-ups for relievers.

2.) Shorter outings.

I decided to draw up a few quick queries on the matter, starting with the history of same-handed match-ups.

I began by querying the Retrosheet database for the percentage of match-ups where the batter and the pitcher were of the same handedness dating back to when this information was first recorded in 1960 (I eliminated from the calculation all Plate Appearances where handedness data was not available). Remember a higher percentage here creates an advantage for the pitcher, a lower percentage favors the hitter.

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Originally published: December 4, 2012. Last Updated: December 4, 2012.