FanPost

Random Variance and Regression

I'd like to highlight a really fantastic post made over on U.S.S. Mariner about random variance and regression to the mean. It's about Russell Branyan and other Mariners, but you can fit the names of Braves's players into the discussion and it will still ring true. Dave explains how when looking at small monthly samples it's often just a few lucky hits sneaking through or a few tough luck called strikes that make monthly lines fluctuate so wildly.

For example, here are Garret Anderson's monthly stat lines so far this season:

In April, 26 PA, 200/231/320, 208 BABIP, .04 K%, .04 BB%

In May, 93 PA, 286/312/369, 299 BABIP, .12 K%, .05 BB%

In June, 90 PA, 306/333/471, 333 BABIP, .17 K%, .04 BB%

In July, 58 PA, 309/345/545, 350 BABIP, .22 K%, .05 BB%

The narrative is that Garret Anderson is "finally coming around"; well, that's not really the case. He's been getting more and more lucky on balls in play, while striking out more, and his walk rate not changing. Literally the only thing he's doing better is hitting home runs, and there's no reason to expect him to maintain a 236 ISO over a longer sample when he's never done so in his career.

Garret's year has been a textbook case of regression to the mean; he wasn't as bad as he was in April, he's not as impotent as he was in May, and he's just not as good as he's been in June and July. Anyone expecting those numbers to continue all season are in for a surprise.

This FanPost does not express the views or opinions of Talking Chop.

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