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August 4, 2004

Lies, Damned Lies

The Legend of Wily Mo

by Nate Silver

I recently heard through the BP grapevine that a prominent figure within the Reds organization has been giving us a hard time about Wily Mo Pena's PECOTA projection. The problem, it seems, is not that the initial projection was too optimistic--Pena's current .285 Equivalent Average matches his 60th percentile projection--but that we were too quick to disclaim it before the season began. Pena's projection, in fact, became something of a running joke within the group, and just as surely outside of it.

I think it's a mistake to assume that your tools are smarter than you are. At the same time, the advantage of an objective system of measurement--be it VORP, PECOTA, or, hell, SAT scores--is that it prevents you from being fooled by biases that you might not even be aware of. Prior to this season, the name "Wily Mo Pena" conjured up an image of a young, chubby ballplayer with terrible plate discipline and a goofy name, someone had been a "prospect" for seemingly forever (a friend of mine drafted him in a Scoresheet Baseball league way back in 1999), and who was only in the big leagues as the result of an ill-advised contract that had been conceived years ago. It's easy to be dismissive of this sort of player; it seems like there are hundreds of them who have come and gone over the years. At one time or another, we've all been fooled by a big performance in a hitters' park by a player repeating the Southern League, or a hot run by an old rookie during a September cup of coffee.

But PECOTA saw some things that it liked in Wily Mo Pena, some strengths lurking amidst all the negatives. Let me talk a little bit about those strengths; why did PECOTA like Pena so much better than any rational observer might have?

1. Power and youth. PECOTA pegged Pena as having isolated power equivalent to that of an average major leaguer. This is, more or less, a straight application of the equivalent statistics prepared by Clay Davenport and Keith Woolner. We can come up with an Equivalent Isolated Power (EqISO) statistic for Pena by subtracting his Equivalent Batting Average (EqBA) from his Equivalent Slugging Percentage (EqSLG). Here were his figures at various levels over the three seasons previous to this one:

   
   Level                    AB     EqISO
   2001 Dayton  (A)        511      .179
   2002 Chattanooga (AA)   388      .147
   2003 Louisville (AAA)    51      .269
   2003 Cincinnati (MLB)   165      .143
   2003 Combined           216      .173
   MLB AVERAGE                      .170
   

   A league average EqISO figure is .170. Pena did a little bit better than that as a 19-year-old in A ball, a little bit worse as a 20-year-old in Double-A, and matched the .170 almost exactly when combining his 2003 performances at Louisville and Cincinnati. One thing that's worth remembering is that, if we're seeking to represent power production, isolated power is a far more reliable metric than slugging average is. Pena hadn't hit for good batting averages, and that's a bad thing, but we shouldn't allow the low slugging averages those BAs create cloud our perception of his power.

   A player who demonstrates an ability to hit for a major-league average level of power at the age of 21 has a very good chance to have a solid major-league career, unless he has absolutely nothing else going for him. (Pena doesn't have a hell of a lot else going for him, but PECOTA found a few other things that it liked; we'll get to that in a moment.) The reason is that power ability continues to develop rapidly throughout a player's early twenties in a way that the other statistical categories do not. (Walk rate also increases throughout a player's career, but doesn't show the rapid growth that power output does.) If a player is capable of hitting for a league-average level of power at age 21, there's a very good chance that he'll be capable of hitting for an elite level of power by the time that he's 24 or 25.

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