Gleeman: Comparing PECOTA projections vs. Vegas

From SABR member Aaron Gleeman at Baseball Prospectus on March 1, 2018:

Last year around this time, I compared PECOTA’s projected team records with Bovada’s over/under team win totals. For nearly half of all teams—14 of 30, to be exact—PECOTA and Bovada were within 2.5 wins of each other, which I defined as being in agreement for these purposes. That left 16 teams on which they did not agree—PECOTA was high on eight and Bovada was high on eight. Of those 16 teams, PECOTA chose the correct side of the over/under line on 11 of them. In other words, if you’d have relied on PECOTA to make over/under bets on those 16 teams and placed, say, $100 on each bet, you’d have gone 11-5 and won $550.

Not bad, and certainly good enough to track this whole thing for another season. However, this time around I ran into a problem: PECOTA and Bovada agree too much. They agreed plenty last year too—I even wrote about some of the reasons why—but this year the two sides are within 2.5 wins of each other for 21 of 30 teams. Perhaps this is more evidence to support Rob Mains’ theory that PECOTA actually sways the public, and in turn odds-makers, moving the over/under lines toward the projections. Maybe it’s evidence that our projection system and their projection system are both pretty damn good. Or it could be random. Whatever the case, only nine teams are in the disagreement pile for 2018.

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Originally published: March 1, 2018. Last Updated: March 1, 2018.