Arthur: Regression stalks the suddenly overachieving

From SABR member Robert Arthur at on November 5, 2014:

With the World Series over, we transition now into the offseason. For fans of struggling franchises and championship contenders alike, now is the time to dream of ways one’s team might be improved. This year’s free agency offers some tantalizing bargains mixed in with some dangerous pitfalls, none more hazardous than 2014’s surprise breakout players.

It’s important to keep in mind, no matter your team, that a free agent pickup is a contract for a player’s future services, as opposed to their past record. It is easy to conflate the two, in part because past performance is the best, most useful guide for predicting future success. The key is that all of a player’s past performance must be considered, and not just the most recent at-bats.

Humans are ever-eager to disregard the more distant past, especially in service of building a “breakout” narrative. Consequently, whenever a player shows marked improvement at the plate, it’s tempting to believe that they’ve reached a new normal, a plateau of increased skill from which they won’t soon fall. This belief is an insidious example of recency bias, whereby we ascribe too much importance to the most recent of our observations. Meanwhile, our brains pretend that years of perfectly valid previous data can be discarded.

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Originally published: November 5, 2014. Last Updated: November 5, 2014.