We’re less than two full years removed from Barry Bonds‘ somber, strange, and soulless quest to break Henry Aaron’s lifetime home-run record. It was a spectacle that most sports fans-even the few like me who were relatively sympathetic towards Bonds’ plight-would go to great lengths to avoid having to experience again.

Unfortunately, it appears that history may be preparing to repeat itself. Alex Rodriguez has already hit 553 home runs, by far the most ever for a player having just completed his age-32 season. He needs only 203 more to surpass Aaron, and 210 to best Bonds. Rodriguez has hit an average of 42 home runs per season since joining the New York Yankees in 2003, and if he maintains that pace, he’ll overtake Bonds’ mark on the last day of the 2013 season. Being under contract with the Yankees through 2017, he seems to have plenty of time to spare.

But player-haters can rejoice: Rodriguez breaking the career home-run record is nowhere near a foregone conclusion. It boils down to that fine print that you ignored when you invested your daughter’s college fund in Citibank stock a few years ago: past performance is no guarantee of future results.

Rodriguez has certainly been among the best players in baseball over the past couple of years. And chemically enhanced or not, there are a number of indicators that would ordinarily be favorable toward his continuing to perform well. Among them:

  • All-around Athleticism:
    Rodriguez is far from a one-dimensional player. At an age when most guys refrain from challenging themselves on the basepaths, he still averages about 20 stolen bases per year. He plays a fairly difficult defensive position, and he plays it well. He’s a complete hitter, able to draw walks and hit for average as well as aim for the fences. Generally speaking, multi-dimensional players age better than uni-dimensional players.

  • The Benjamin Button Principle:
    This is the concept that the beginning of a player’s life sometimes resembles the end: guys who begin their careers with a bang tend to end it that way. Rodriguez, who by the age of 20 was already arguably the best player in baseball, started his career as did few others in history, and he has a better-than-usual chance of finishing it that way.

  • Perverse Incentives, Part I:
    Rodriguez stands to earn a $30 million bonus if he can break the home-run record. As he gets closer, those are 30 million reasons for him to extend his career until he does, rather than considering early retirement.

On the other hand, another set of indicators imply uncertainty in Rodriguez’ future:

  • The Aging Curve:
    The steepest part of the aging curve-when a hitter experiences the most manifest decline in his abilities-tends to come between ages 32 and 34. Rodriguez, who turned 33 last July, is now about half-way through that period, and he hasn’t come away completely unscathed: A-Rod hit 30 home runs in the first half of the 2007 season and 24 in the second half, and then 19 home runs in the first half of ’08 and only 16 after the break. That could just be a fluke-or it could mean that he’s already begun a fairly steep downward trajectory.

  • Injury Risk:
    Although Rodriguez has generally been the picture of health, that trend somewhat reversed itself in 2008 when he missed 24 games, the most in any season since 1999. Injury problems can sometimes be compounding, especially when a player reaches his mid-30s. There is also some anecdotal evidence that players who have experimented with steroids are more inclined to have chronic injury problems.

  • Perverse Incentives, Part II:
    Unless he was investing with Bernie Madoff, Rodriguez already has all the money that he’ll need for life, and it’s highly unlikely that he’ll ever be on the market again. Most of us, given a guaranteed salary for the next nine years that requires us to do nothing other than show up and put on a uniform, might become somewhat lackadaisical in our work habits. Many professional athletes are different-but others aren’t.

The favorable and unfavorable indicators are each reflected to some degree in Rodriguez’ series of PECOTA comparables. His list includes many Hall of Famers, such as Dave Winfield, George Brett, Frank Robinson, Reggie Jackson, Tony Perez, and Hank Aaron himself, who were all elite athletes late into their 30s or early 40s.

However, it also includes some other players whose careers did not end all that gracefully. First are the guys who succumbed to injury, like Jeff Bagwell and Albert Belle. Next are a few players who, like Rodriguez, were known or suspected to have used performance-enhancing drugs: Sammy Sosa is A-Rod’s top comparable, for instance, and Ken Caminiti is his fourth. Finally, there are players like Ryne Sandberg, whose skills simply atrophied sooner or more suddenly than expected.

I took Rodriguez’ top 20 PECOTA comparable players and averaged their performances over each remaining season of their careers. Actually, the process is a little more complicated than that (each comparable’s performance was adjusted for his park and league context, as well as his previous track record, and we had to make an accommodation for guys like Manny Ramirez, who made A-Rod’s comparables list but have yet to conclude their own careers). The basic idea though, is simple: comparables like Frank Robinson, who aged well, have a favorable impact on Rodriguez’ forecast, and players like Caminiti have the opposite effect.

Alex Rodriguez’ PECOTA-Projected Home Run Totals:

Year     HR
2009     33
2010     30
2011     27
2012     25
2013     18
2014     16
2015     12
2016      8
2017      4
2018      3
2019      1
Total   177
Career  730

PECOTA’s best guess is that Rodriguez will run out of steam after the next three or four seasons and finish with 730 lifetime home runs, leaving him just shy of the marks established by Aaron and Bonds. Of course, there is a great deal of uncertainty in this estimate: if Rodriguez follows the path charted by Aaron or Frank Robinson, he could finish with well in excess of 800 home runs (and possibly as many as 900). On the other hand, if he draws Albert Belle’s ping-pong ball, he might not even top 600. Overall, the system puts Rodriguez’ chances of surpassing Aaron at only about 40 percent, and of passing Bonds closer to 30 percent.

One needs to remember that the way that Aaron and Bonds finished out their careers was far from typical. At least as common are folks like Jimmie Foxx (before Rodriguez, the fastest player to 500 home runs), who hit just 34 home runs after turning 33. Only about a dozen players have hit 200 or more home runs from their age-33 seasons onward; Bonds and Aaron are the only two to have hit at least 300.

In other words, Rodriguez still has his work cut out for him if he intends to catch them. Say what you will about his past performance, but for him to make it across this finish line would still represent a remarkable accomplishment.

Thank you for reading

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Nicely done Nate ....

One thing that is a wild card in your equation ... how will the new Stadium play? Will A-Rod\'s homer rate change with the new venue? Will he turn into Bobby Murcer at Shea Stadium? (doubtful, but still possible).

Even with similar dimensions, who knows if the wind currents will carry fly balls differently than in the old stadium.
The older players whose careers fell off the charts due to injury are poor comparables; because of preventative care, surgery, and rehabilitation we are in a different world now. If those players had been born in 1980 their career trajectories would have been entirely different.

Ergo, A-rod\'s chance of breaking Bonds\' record are substantially higher than 30%.

Right, preventive care... and PEDs.
Preventive care is one of the HUGE factors why players use PEDs. They can stay on the field by recovering from or avoiding injuries. Obviously we know steroid build muscle to a mass that the human body can\'t usually handle, thus injuries. On one hand I say, let them all use, who cares. On the other hand, I do care, I am really torn over all this.

Great point, and one that seems to be ignored by BP when it comes to projections on historical comparables. It is reasonable to believe that advanced medical and training methods would extend careers that are worth millions. The concept of \"aging well\" is used often, but the conditions that lead to a player aging well seem vague or scantily described. It is mostly attributed to some natural, physical ability like being athletic, but maybe what the player did to his body is also a factor?

Of course, we would need sufficient data on modern aging to know whether there is something substantial behind the conjecture. How about a study?
That\'s crazy talk, Nate. PECOTA has to break down for players of A-Rod\'s stature, because it can\'t account for his extraordinary motivation to break the record, or for the fact that he\'ll be able to get deals even if he does break down (see Ken Griffey Jr).

I hope you\'re right - I\'m a Red Sox fan - but you\'re not.

Eh, I\'m just cranky because you told me to take Mickey Rourke in my Oscar pool.
A-Rod\'s mental toughness (or lack of) might also come into play. He hasn\'t done all that well under the pressure of postseason play and I suspect he is going to be facing a hostile fan base on the road that has only been seen by guys like Jackie Robinson and Barry Bonds.
Ooh! Is LDL back as a regular feature, Nate?
The election is over, the inauguration is over. More BP, less 538 Nate. Please?
(a) Nate, didn\'t you do a career HR projection for A-Rod a few years ago (was it on BP, ESPN, or SI)? Can you compare the actual HR producdtion to your projection then?

(b) Also here\'s an interesting PECOTA exercise, which I suggested on last week\'s article by Steve Goldman, in response to a reader who wanted to look at \"average\" players:

Here\'s another exercise that could be done that might yield similar results to what you\'re interested in: which player at each position shows up most frequently as a PECOTA comparable? Presumably those individual should be the most \"typical\" or average at their positions historically (in Nate\'s PECOTA database).

Please consider this.

Give us a \'Comparable All Stars, 2009\' (maybe top 3 to 5 at each position?).\" Presumably there would be a bit of a longevity bias -- longer-career \"average players\" have a greater chance to become comparable all-stars. But mediocre longevity deserves recognition!!!
Here\'s Nate\'s assessent from 2005 about the prospects of Pujols and ARod overtaking Aaron. Kind of interesting to reflect back on the assumptions then.
Thanks for descending from your cloud to give us this, Mr. Big Shot.

Just kidding. But seriously, I hope that you have time for LDL more often - I love your work here.
\"Rodriguez already has all the money that he\'ll need for life\"

I hate this argument. You don\'t know what he wants to do with that money. Maybe he wants to start a baseball league in Puerto Rico. Maybe he wants to buy the Florida Marlins. Maybe he wants to run for President.
It\'s unlikely he\'d be a worse owner than Loria in Florida...
Can PECOTA really create meaningful comparisons for atypical players like A-Rod? There aren\'t really a ton of middle infielders that have hit 50 HR and 20 SB in a season. Sammy Sosa didn\'t turn into a decent player until 1996 or so, while A-Rod started producing pretty much out of the gate as a rookie. Nor did Sosa immediately get the plate discipline to walk a whole bunch.
This was my issue with the idea. I have a feeling (perhaps unfounded) that PECOTA gets worse and worse with dealing with extremes. Beyond things like variance (which obviously you can\'t fault PECOTA for, since variance is by definition random) someone like ARod is such a unique player (as most no-doubt HoFers are) that they become more difficult to classify.

Especially once you strip away those (as jtwranch noted) who broke down due to forces that wouldn\'t end a career in the modern game, even someone like Foxx (since ARod isn\'t an alcoholic) who is left? What other infielder hits for power like ARod, has that speed this late in his career, and has that level (admittedly now declined, but still) of fielding talent?
I can\'t believe we\'re only a few years away from ARod\'s somber, strange, and soulless quest to break Barry Bond\'s lifetime home-run record.

Should be a fun!

One more small factor in A-Rod breaking Bonds\'s record: What if Bonds gets more HRs himself? I\'d bet he\'s going to beat his rap and he can still DH and pinch hit. Will anyone sign him?
Ah, PECOTA, ever the pessimist. I\'ll gladly take the over on 730. Hell, I\'d take the over on 800.
Reminds of me when PECOTA was pessimistic on the 2007 Rays...
The PED effect is the variable that can\'t be judged here. I know BP is sacred \'put our fingers in our ears/heads in the sand\' territory regarding PEDs, but the reality is it is confirmed that A-Rod attempted to enhance his performance with PEDs and the circumstantial evidence is that this continued beyond the years to which he has admitted. How can this be figured in? It can\'t, of course. We don\'t know the positive effects of PEDs; we don\'t know their injury recovery/avoidance effects; we don\'t know precisely when he took them in the past; and we don\'t even know if he might continue to take them given, of course, that he is a mini-corporation who can easily afford his own medical dept to keep him ahead of the testing curve (one could argue that his incentive to do PEDs will be particularly strong this year, in order to show previous years\' performances were not PED-impacted). But one thing we do know: it\'s a variable for which it is difficult to take account.

It is, of course, easier to live in la-la land and say this is a non-issue. But since this is a Nate Silver piece I\'ll be a bit political and steal a phrase from the last campaign: i.e., I\'m all for joining the reality-based community. This variable has to be recognized even if it\'s effects can\'t be measured and makes Pecota\'s projections all the more uncertain.

Wait, hold the phone here. You want to make a few claims here that we can\'t trust what previous historical comps have done past A-Rod\'s age because he a) took PEDs in 2001-2003, b) may have taken them in 2004-2008, and c) might even take them in 2009-beyond. From this you deduce that you can\'t take the variable into account.

But you can, you know. First off, if PEDs do not enhance performance in any meaningful way, then the variable of whether or not someone is taking them is moot. So the assumption here you\'re making is that there IS an effect. Which is fine. Because if there IS an effect, that\'s moot, too - because PECOTA has been blithely making predictions on A-Rod\'s stats for years now without knowing about his PED use, and the fact that we can point to some choice he made back in 2001-2003 as confrmed and cannot confirm the decisions he made on the same issue in 2004-2008 and beyond doesn\'t change the model in any way. What DOES change the model from year to year, I would presume, are systemic inaccuracies that Nate tweaks to improve the overall accuracy of the model. Those tweaks encompass billions of variables, from whether A-Rod is juicing, to whether Prince Fielder is on the Skinny Bitch diet this year, to what time Cole Hamels goes to bed each night. Think about it: PECOTA has no idea what Joel Zumaya does in his spare time, but the moment his Guitar Hero addiction impacted his playing time, his forecast changed by a little. Whatever outside influence affects the on-field performance, PECOTA will extrapolate purely from the onfield results. So by averaging the futures of the 20 people in baseball history who had pasts most like that of A-Rod, you get a ballpark figure of the range of possible futures he might have, even though there were hundreds of variables specific to each of those players that you can\'t control. What\'s one more?

What does seem certain is that it is unlikely that he has been clean in the past and would start juicing now, after a positive test. Which means that if PECOTA WAS lax in figuring the effect of PEDs, his chances of breaking the record may be even less than the computed figures of 30 percent. Which means that A-Rod still has his work cut out for him.

Which is kinda Nate\'s whole point.
Im always surprised when Bill James\' formula is not used or at least referenced when projecting stats. I do not know if there is any research into the accuracy of it but the method is better than averaging 20 top comparables in my opinion. There is a free program you could once download which I currently can not find but the formula is available all over the net as is this nice little tool on ESPN:
As others have commented, PECOTA seems to be less accurate projecting careers of extreme players, including players like A-Rod, so good at playing baseball that he is among the very few best ever to have played the game. It\'s tough choosing 20 comparable players to A-Rod because there aren\'t twenty other position players as good as A-Rod at his age. Picking comparables means minimizing ways that other players are inferior.

That said, some of the players listed as A-Rod\'s best comparables aren\'t reasonable choices because they\'re slow-moving sluggers prone to swift decline, not excellent shortstops who moved to third base to make room for another HOF-caliber player. Greg Luzinski is, to me, the most laughable comp: he had earned -132 FRAA over a career at left field and first base by age 29, and he played his last four years as a DH. Saying that a 32-year-old Greg Luzinski is the twelfth-best comparable to a 32-year-old A-Rod is, to my mind, absurd. Yes, Luzinski played just one more year and retired after a disappointing season at age 33. But he\'d only played 1.2 games on defense in the last four years before retiring. That\'s not comparable to what A-Rod has done at third base his past few years.

There are other among A-Rod\'s comps who were declining slow-moving sluggers. Albert Belle fits into this group: while his career ended swiftly, lots of slow-moving corner outfielders and corner infielders see their careers end swiftly in their early thirties. Jimmie Foxx, mentioned in the article, fits that mold as well.

The trouble with A-Rod is that his defensive comparables were lesser hitters. Still, there are four of twenty comparable players who were reasonably similar to A-Rod in the field through their careers: Brett, Grich, Sandberg and DeCinces. (Caminiti was clearly a lesser fielder.) Of those four, Sandberg oddly retired after a bad start in 1994 at age 34 before returning for two years at age 36, Grich and DeCinces had reasonable decline curves, and Brett was still a 7.1 WARP player at age 37 and a 19-HR hitter at age 40, when he retired. It\'s tough calling Sandberg\'s retirement choices \"comparable,\" but he, DeCinces and Grich all ultimately retired at ages 36-37 because they needed to do so. Brett, probably the hitter of this group most like A-Rod, was still an MLB-caliber player when he chose to retire at age 40, and he was hitting home runs at better than half his peak rate from his prime. This article has A-Rod hitting just eight HR at age 40: discounting the lesser fielders (Luzinski, Belle) and the lesser hitters (Sandberg, Grich, DeCinces), and maybe discounting the known juicers (Caminiti), A-Rod\'s odds look better.

I\'d caution, though, after all of this writing intended to bolster A-Rod\'s case, there\'s one thing that still troubles me. If you check A-Rod\'s best comparables at Baseball Reference, there\'s one from the top ten who\'s better than any BP comp in many ways: Rogers Hornsby. Both Hornsby and A-Rod were probably the second-best hitters of their era, each surpassed only by a corner outfielder who defied the standards of their times.

That said, Hornsby had just one more MVP-caliber season left at A-Rod\'s age, his 14.9 WARP 1929 season at age 33. He injured his foot badly in 1930, ruining the season. He came back for a 7.6 WARP 1931, but then he accrued just 2.8 WARP and just six home runs over the next seven years before retiring. His last MLB-caliber year was at age 35, just one year earlier than Grich, DeCinces and Sandberg all found themselves nearing retirement.

While I\'ve questioned the validity of PECOTA comps for a player as good as A-Rod, maybe the answer of \"probably not\" remains valid for this question all the same.

While I agree with the overall content of your comments (including the Hornsby comp), I have to quibble with:

There are other among A-Rod\'s comps who were declining slow-moving sluggers. Albert Belle fits into this group

I think your opinion of Albert is coloring your memory. He was 17-3 in stolen base attempts in 1999, his last full season. That doesn\'t look like a \"declining, slow-moving slugger\" to me.
Certainly a valid counterpoint, Dr. Dave.

I\'d offer that Belle was 10-8 in stolen base attempts the two years prior to the one you cite, and that he was 0-5 the next year, his last. His career 3B/2B ratio was 21/381, 5.5%, in an era when league norms were roughly double that. His range factors and Davenport Fielding Rates were average for the corner outfield positions he played, and corner outfielders aren\'t always noted for speed.

I still consider Belle a slow-moving slugger, just as I consider Jason Varitek the same way despite his 10-3 stolen base success rate in 2004. Belle had three or four good seasons stealing bases, not just one, so your point that he could occasionally have real value as a base stealer is valid--it\'s just that I don\'t see occasional success as a base stealer offsetting all the other things, including some bad seasons trying to steal bases, that make me consider him a slow-moving slugger.
Belle also retired due to an unusual degenerative hip condition, which might have added to the \"slow-moving slugger\" idea -- and doesn\'t help PECOTA\'s case in calling him comparable to A-Rod.
Even Griffey, with his diminished skillset, has not been projected to hit less than 20 home runs. I don\'t see how A-Rod can be projected to hit less than 20 from ages 36-43. Just the sheer cost of his contract means he could miss a season from rehab and still be allowed to play out the rest of it. People just don\'t retire from injuries as much as they used to.
I was struck by A-Rod\'s comps when I read the annual. Caminiti? Yeesh.
Great article Nate.

Saw this on this morning and reading the comments from the people the absorb everything Joe Morgan and every other terrible analyst says was probably the highlight of my day (as sad as that is).

Anyone with insider access that wants a laugh should check it out.
What\'s up with the same articles being here and at ESPN looking at buying BP or something?
That would be terrible and the end of BP as we know it.
What\'s the steepest part of a pitcher\'s age curve?
I don\'t know if PECOTA deals with this effectively or not, but my socks don\'t match.
Do we have enough historical data to back test Pecota projections of first ballot hall of famers? I\'d like to know if Pecota would have forcast those extraordinary careers. For example, what would Pecota have forecast for the Babe\'s last ten years?

Baseball is now a mature enterprise. That means that players, trainers, coaches are all more tightly bunched at the top of the learning curve. There should be much less variance between all of those individuals. It also means that prediction systems like Pecota will become more and more accurate. (A hundred years from now we won\'t even have to actually play the games!)

It seems that it would be useful to compare the average rate at which players have declined over the last 25 years with the rate at which they declined 50, 75, and 100 years ago. That would give us some idea about how valuable those old comparables are for predicting a modern player\'s last few years. The guess here is that there will be a significant difference in the rate of decline.

One hundred years ago the incentive to play late into a career was not nearly as great as it is today. Players were not making the kind of money they make today-even adjusted for inflation. The travel was much more brutal; and probably took a greater toll on a player\'s home life. That\'s without even getting into the medical, coaching, diet and training issues.
That\'s an interesting question, jtwranch. In hindsight, how well can PECOTA predict the careers of retired players. Did a player like Babe Ruth over or underperform his career projections? What about someone more recent like Ken Griffey Jr or Don Mattingly?
PECOTA\'s home run total for the rest of his career seems ridiculously low. Will there be a downturn, sure, that\'s very likely, but come on. He\'s suddenly only going to hit 8 home runs (and then 4!) at the end of his Yankee contract? That doesn\'t look like it\'s predicting a natural decline... considering A-Rod\'s ability, that\'s predicting a complete and total implosion where he suddenly loses all of his strength. Unless PECOTA can see the future and knows he\'s going to come down with a debilitating disease that saps all of his strength (or is predicting an injury 9 years from now, which seems quite out there), I don\'t see any chance of that.

I also don\'t see this \"home run decline\" over the past two years to be legit. In 07, he had 54 less plate appearances in the second half, and he also was having his best season ever. If anything is the anomaly, it\'s 30 home runs in the first half, not the 24 in the second half. In 08, he missed time with the injury and still hit 35 home runs...while he certainly wasn\'t on pace to repeat his 2007 season, I\'d hardly call that a huge dropoff, especially considering in 06 he hit 35 home runs in a full season (and he hit 36 in 2004 for that matter). It\'s not impossible it was due to a decline, but that certainly wouldn\'t be my first explanation, not considering the injury and the fact that he\'s not consistently hitting 50+ home runs every single year.
I, for one, hope he doesn\'t even crack 700.
The list of ARod comparables show why steroids make PECOTA a HOUSE OF CARDS.

Since SOSA is Arod\'s closest comp, do we have to assume that Arod needs to continue taking steroids to match Sosa\'a numbers?

Or do we assume that Sammy had the greater benefit of steroids up to this age in his career and his steroid-inflated numbers really aren\'t a comp for Arod (if you discount any benefit for arod thus far from them).

The point is that all these comps are USELESS unless you know how the numbers were generated with respect to steroids.

Is a \'clean\' Dave Winfield or a \'dirty\' Sammy Sosa or Ken Caminiti a better comp? Simply comparing numbers is PURE FOLLY.
That\'s kind of dangerous territory, sbnirish77. There are also a lot of possible explanations for why Sosa\'s career tanked. He hurt his toe, he got beaned in the head, he got caught with a corked bat and he got in Dusty Baker\'s doghouse. It\'s hard to tell what factors ultimately contribute to one\'s success or failure.
Nate could have used this article to state his position on the influence of steroids upon PECOTA but sidestepped the issue. Eventually he\'ll have to come to terms with the subject.
Enough of this \"story\". With all the crap we have to listen to every day who wants to listen to more of this negativity. Let\'s keep our beloved baseball the safe haven from everyday bull that it should be and not sink to \"People\" magazine levels. Play ball!!!