Regression Toward the Mean: What It Is

If you’ve ever read several articles about fantasy baseball analysis, then chances are you’ve met the phrase “regression toward the mean.” This is often interchangeably used with the phrase “player BABIP bound to regress.” Do you know what it means?

Batting Average on Balls in Play

In baseball statistics, BABIP or batting average on balls in play is used to measure how many of the batter’s balls which come in play has gone for hits. It can also measure how many balls which come in play against the pitcher has gone for hits with the exclusion of home runs.

The BABIP average in the major league is .300. Different factors may influence a player’s BABIP, so it can be lower or higher than this.

How Is This Related to Regression Toward the Mean?

Let’s take 100 baseball hitters for example and say that 50 of them has a BABIP greater than .300 and the other 50 has a BABIP lower than .300. The former players may have benefitted from good luck while the latter suffers from bad luck. Why?

Statistically speaking, there’s a 3-in-10 chance that a player can get a hit of .300 BABIP. Even in the group of 50 with high BABIP, you can still expect BABIP to be .300 in the next season. The same can be said with the group of 50 with low BABIP. This is what “regression toward the mean” means.

In other words, regression toward the mean refers to the phenomenon where a variable that’s extreme on the first measurement (high or low BABIP) tends to be closer on average (.300) during the second measurement. Likewise, if the measurement is extreme on the second measurement, it may be closer to the average on the first measurement.

Regression is applicable on both overperforming and underperforming players. In both cases, you can expect their BABIP to regress toward the mean on the second or third year, if not on the first.

It’s Not a Natural Law

Regression toward the mean does not necessarily imply that those who were lucky in the first season will experience bad luck in the next. What you can expect is that they’re bound to cool off and move towards their average.

You must know that regression toward the mean is not a natural law. It’s simply a statistical tendency. Sometimes, it will take a long time before it happens.

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