March 26, 2009

Lies, Damned Lies

Stress Tests

by Nate Silver

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Stress tests are not just for cardiac patients and big failing banks: we can also apply them to baseball players. What if Tiny Tim Lincecum were really Tall Tim Lincecum? What if Albert Pujols was older than purported? What if we took a right-handed pitcher and magically turned him into a lefty?

PECOTA, Baseball Prospectus' forecasting system, can answer some of these questions. PECOTA looks not only at a player's prior statistics when projecting his performance, but also some of his physical attributes: his height, weight, handedness, age, and so forth. These factors affect the selection of a player's comparables, and in turn can affect his forecast, somewhat breaking down the barrier between attributes traditionally associated with statistical analysis and those associated with scouting.

For our first trick, for example, let's do what Mother Nature and Father Fielder couldn't do and make Prince Fielder skinny, slimming him down to a svelte 165 pounds:

               PA   AVG/ OBP/ SLG  HR  RBI  Top Comp
Fat Prince    661  .286/.381/.528  33  107  Boog Powell
Little Prince 660  .277/.367/.511  32  103  Will Clark

Somewhat counterintuitively, PECOTA does not see Fielder's weight as a problem. On the contrary, it tends to like beefier and brawnier hitters who can put more oomph into their swings-though to be fair, it has no way to distinguish good weight (muscle) from bad (double cheeseburgers, or in the case of the vegetarian Fielder, double Boca Burgers). What's surprising is that most of the hit that Fielder takes to his forecast is in his batting average rather than his power output. But we should caution that we have only made Fielder smaller-not faster. In real life, a slimmed-down Prince would be able to run the bases a little better, and that would mean some extra-base hits, probably at the expense of a few home runs.

Next, we'll take Dustin Pedroia and make him 6'5" instead of his generously listed official height of 5'9" (I'm 5'9", and I'm pretty sure that I'd have a couple of inches on Pedroia if you put us side by side).

                 PA   AVG/ OBP /SLG  HR  RBI  Top Comp
Petite Pedroia  649  .304/.366/.449  12   74  Ron Hunt
Bigfoot Pedroia 656  .310/.376/.466  14   78  Davey Johnson

Once again, bigger is better. We have Pedroia's batting average going up, and those aren't singles he's getting-they're extra-base hits. We also have Pedroia taking a few more walks, which is mildly surprising, since shorter players have a smaller strike zone to control. However, drawing walks at the major league level is as much about commanding respect from pitchers as it is about commanding the strike zone; if Pedroia were more of an extra-base threat, he'd get pitched around more.

We can perform the same exercise with pitchers. Not that Tim Lincecum needs much help, but what if he were 6'6" instead of five-foot-something? (To keep this somewhat realistic, we'll also increase his weight to 190 pounds from 170).

           IP     W-L    ERA  BB   K   Top Comp
Tiny Tim  205.2  13-9   3.29  78  221  Johnny Antonelli
Tall Tim  206.2  13-10  3.30  81  220  Roger Clemens

This turns out to make no difference at all. Body type, generally speaking, has less predictive importance for pitchers than it does for hitters. Taller pitchers may have their strikeout numbers hold up better over time, and can sometimes generate more ground balls by throwing on a downward slope. On the other hand, their deliveries are less compact, and they may walk a few extra batters (as the forecast has Lincecum doing here). With a couple of exceptions depending on repertoire (you won't find many short sinkerballers), the impacts are generally a wash.

Let's now turn CC Sabathia into a normal-sized, baseball-playing human being: 6'2" and 210 pounds.

         IP     W-L   ERA  BB   K   Top Comp
XXL CC  232.1  16-9  3.31  53  201  Don Drysdale
L CC    232.2  16-9  3.33  54  200  Hal Newhouser

With the caveat that Sabathia is so unusual that PECOTA doesn't know whom to compare him with in the first place, we once again do not see a significant impact; weight is even less important for pitchers than height. There is one minor exception, however, in Sabathia's case. While the forecast above shows his performance only for next year, PECOTA also forecasts trends up to seven years in advance, and it sees the 210-pound Sabathia as being somewhat less injury-prone, making 181 starts over the next seven years instead of the real Sabathia's 168.

Does handedness matter? Justin Duchscherer is someone who has achieved success against type; his finesse repertoire is something you'd ordinarily associate with a lefty, but instead he's a righty. Would actually being left-handed help him?

                 IP     W-L   ERA  BB   K   Top Comp
Duchscherighty  156.2  10-8  3.97  42  101  Sonny Siebert
Duchschelefty   161.0  10-8  3.88  43  104  Bob Ojeda

In this case, we do see a small improvement in Duchscherer's performance; he shaves about a tenth of a point off of his ERA while striking out a few more opponents. Generally speaking, however, the stereotypes that scouts like to associate with left- and right-handed pitchers are exaggerated in their importance, and more non-power righties like Duchscherer deserve a chance.

Where we do see a bit more action is when we play with a player's age. Let's take the Cardinals' best prospect, Colby Rasmus, and age him by two years, so he'll be 24 this season rather than 22.

                  PA   AVG/ OBP /SLG  HR  RBI  Top Comp
Rasmus (Age 22)  535  .248/.330/.417  16   59  Ed Kirkpatrick
Rasmus (Age 24)  454  .229/.306/.382  12   49  Orsino Hill

Those two years make a huge difference, knocking about 60 points off of Rasmus' OPS, cutting into his playing time, and basically transforming him from an elite prospect into a fourth outfielder. The impacts are even more profound if we look further out into the future, as the two lost years make Rasmus' UPSIDE forecast, PECOTA's best gauge of a player's long-term value, decrease by roughly two-thirds. Hitting prospects only have so many years to develop-they're usually done improving by the time they turn 25 or 26-so depriving a player of two years of development time changes everything. Baseball players know this, and so do baseball teams, which is why some players, particularly in Latin America, feel compelled to lie about their age.

For established players, however, age makes somewhat less of a difference. Suppose, for example, that Rasmus' teammate Albert Pujols, a player who has sometimes been rumored to be older than listed, turned out to be 31 rather than 29:

                  PA   AVG/ OBP /SLG  HR  RBI  Top Comp
Pujols (Age 29)  663  .339/.443/.609  35  117  Frank Thomas
Pujols (Age 31)  668  .337/.439/.595  33  113  Frank Robinson

While this is not a good development for Pujols-he loses two home runs and four RBI-the differences are relatively negligible. It's on the steepest parts of a player's aging curve-which means the beginning and the end of his career-when a change in age would tend to make the most difference. The difference between a 23-year-old prospect and a 21-year-old prospect is enormous; the difference between a 29-year-old player and a 27-year-old player much less so.

One caution about the forecasts you see above: when dealing with these kinds of counterfactuals, we should remember that a difference in a player's physiology would affect not just his future performance, but also his past performance. Dustin Pedroia, for instance, would probably have hit a few more home runs last year if he had actually been 6'5", and since the PECOTA forecasts are based on a combination of stats and scouting attributes, that would further increase the impact of his having added additional height. Thus, these forecasts may tend to understate the impact of these kinds of differences.

Nevertheless, one of the most deeply satisfying things about baseball is that all sorts of men can succeed at playing it: fat men and skinny men, short men and tall, young men and middle-aged ones, men who look like David Wright, and men who look like Kenny Powers. It's fitting, then, since we're a country that prizes equal opportunity, that baseball is our national sport.

A version of this story originally appeared on ESPN Insider .

Nate Silver is an author of Baseball Prospectus. 
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