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Does Switching Leagues Affect Pitching Performance?

When Major League pitchers change leagues, strange things begin to happen.

Jordan Zimmermann, an ace by any measure when he played for the Washington Nationals, became a free agent after the 2015 season before signing with the Detroit Tigers — only to quickly learn that he was not so invincible.

Wei-Yin Chen’s four strong seasons with the Baltimore Orioles gave him free agent swagger — and a hefty asking price — that led him to a spot on the Marlins roster just before the 2016 season. And then he suddenly became very human.

As is often the case, the stories that develop around pitchers as they transition from one league to the next are fed by assumptions, a bit of baseball history, certainly a little bias, and what can only be called the gut instincts of the fan.

Statistics, of course, aren’t infected by ESPN punditry and don’t care what color a uniform is. They are emotionless, sober friends of reason.

The known stats of Zimmermann and Chen — before I got involved — tell you part of the story. A deeper analysis, however, upends the conventional wisdom. Read the rest of this entry »