Printer-friendly

April 18, 2008

Lies, Damned Lies

Adding Birthdays

by Nate Silver

My first thought after I learned that Miguel Tejada was two years older than his listed birth date was that I wasn’t really all that surprised by the news. My second thought was that he just threw away his shot at the Hall of Fame. One of these two thoughts is valid; the other is a little out of place. Let’s take care of the obvious part first. Below is a comparison of Miguel Tejada’s original PECOTA forecast with a new one that we’ve generated by aging him exactly two years and leaving everything else alone:

If you own Miguel Tejada in a fantasy league this year, you can breathe easy—PECOTA had already registered a fairly pessimistic forecast for Tejada, and it only got just the slightest bit more pessimistic as a result of aging him by two seasons. We actually have him drawing a few more walks; it’s not uncommon for players to draw more walks as they age, as they tend to work the count harder once their bat speed declines. However, those walks come at the expense of a handful of extra-base hits. Overall, we have him losing just a couple of points of VORP, and about $1.0 million in MORP—not anything that should cause Ed Wade to break into night sweats.

Were you expecting to see something more dramatic? Keep in mind that there is a little bit of luck involved in PECOTA, since we’re running the player’s forecast based on the performance of actual comparables, rather than some theoretical aging curve. Fundamentally, I would argue that we shouldn’t be that surprised. The reason gets into sort of Bayesian math. We just aged Tejada by two years, but we also aged his previous performances by two years, so it’s not like we’re comparing a 32-year-old who was a good major leaguer at age 31 to a 34-year-old who was a good major leaguer at age 31. We’re comparing a 32-year-old who was a good major leaguer at age 31 to a 34-year-old who was a good major leaguer at age 33—and it’s more difficult to be a good major leaguer at age 33 than at age 31. So, while the circumstances are now less favorable to Tejada, in a sense he has also accomplished a little bit more. It also happens to be the case that age-32 seasons tend to be a particularly accursed point in the careers of certain major leaguers; that's when Dale Murphy’s career imploded. The fact that we now know that Tejada survived that year works somewhat to his favor.

However, there is a little more drama if we look further down the line. That’s when the immediate fact of his recent performances becomes less important in his forecasts, and the fundamental facts about Mother Nature a lot more so. In 2009, we have him losing about eight points of EqA; perhaps more importantly, his attrition rate is much higher, 30 percent versus just 14 percent before. These two factors conspire to wipe about $3.2 million off his MORP. Fortunately for the Astros, 2009 will be the last year of Tejada’s present contract, as his decline tends to get steeper from there: