Here is a good article on this issue by Nate Silver.
By Nate Silver, BaseballProspectus.com
Barry Bonds haters are comforting themselves with the notion that Bonds' hold on the home-run record could be short-lived. Alex Rodriguez is more than 100 home runs ahead of Bonds' pace at the same age, and if he doesn't break the record, there are a number of other candidates such as Albert Pujols waiting in line for their shot at the piñata.
For that matter, is it too soon to count out Ken Griffey Jr.? How about Sammy Sosa? For an answer, we turn to a modified version of Baseball Prospectus' PECOTA projection system.
PECOTA uses comparable players to forecast a player's future. In other words, each potential comparable represents a positive or negative precedent. If Hank Aaron appears on a player's comp list -- someone who was productive well into his early 40s -- that will increase the model's estimate of that player's likelihood of challenging the record. If he gets someone like Eddie Mathews instead, a great player early in his career but who hit just three home runs after age 35, that will harm his projection.
For the purposes of this exercise, we estimated Bonds' final career total to be 782 home runs (see note below for more on the methodology). So here are our 13 contestants, all with significant home run credentials, ranked in inverse order of PECOTA's estimate of their likelihood of challenging Aaron and Bonds.
Thanks for Playing, but No Record for You
13. Vladimir Guerrero
Projected Career Total: 495
Chances of Hitting 500 HR: 41.6%
... 600 HR: 9.6%
... 700 HR: none
Top Comparables: Juan Gonzalez, George Scott, Joe Torre, Dave Winfield, George Hendrick.
Vlad is a hard player to characterize -- his combination of raw power and a low strikeout rate is rather unusual in today's game -- and I'm not entirely certain that the PECOTA comparables do him justice. Nevertheless, he's topped 40 home runs in a season just twice, and his propensity to the occasional injury doesn't help him. At 31, there's just too much ground to make up for the Home Run Derby champ.
12. Frank Thomas
Projected Career Total: 573
Chances of Hitting 600 HR: 22.2%
... 700 HR: none
Top Comparables: Darrell Evans, Rafael Palmeiro, Mike Schmidt, Willie McCovey, Fred McGriff.
At 39, Thomas is the oldest player on this list and therefore the easiest to project; the fewer years a player has left, the narrower his range of potential outcomes, and vice versa. It's fairly impressive that PECOTA thinks Big Hurt might have another 100 home runs left in his bat, but 700 and all numbers north are out of the question.
11. Jim Thome
Projected Career Total: 586
Chances of Hitting 500 HR: 100%
... 600 HR: 38.2%
... 700 HR: 6.8%
... 755 HR: none
Top Comparables: Thomas, McCovey, Evans, Willie Stargell, Ellis Burks.
Thome's career slugging percentage of .564 is a hair better than that of Aaron (.555) or Mickey Mantle (.557), but he's simply missed too much time due to injury, something which isn't likely to abate as he ages (he turns 37 in August). A healthy 2005 and he'd have had a fighting chance at Aaron, though probably not Bonds.
10. Sammy Sosa
Projected Career Total: 647
Chances of Hitting 700 HR: 8.8%
... 755 HR: none
Top Comparables: Tony Perez, Ruben Sierra, Gary Gaetti, Ernie Banks, Gil Hodges.
PECOTA was not impressed with the Rangers' signing of Sosa before the season and it hasn't really changed its tune -- Sosa's comparables are players that hung on too long. Sosa retains above-average power but is costing his club in every other department; he might not run out of time so much as he runs out of people willing to employ him.
The Pretenders
9. Ken Griffey Jr.
Projected Career Total: 648
Chances of Hitting 600: 95.6%
... 700 HR: 11.4%
... 755 HR: 0.5%
... 782 HR: none
Top Comparables: Dave Parker, Andre Dawson, Eddie Murray, Fred Lynn, McGriff.
Griffey has rehabilitated his image as one of the all-time greats with a fine season, but he hasn't hit 40 home runs in a season since 2000, and he'd need several such seasons to make a serious run at Bonds or Aaron. Perhaps my least favorite question is whether Griffey, now 37, would pose a more serious threat at the record if he'd never been injured. Although the answer to that question is obviously "yes," you can construct a similar contingency for just about any player in history. How many more home runs would Babe Ruth have hit if the Red Sox hadn't fiddled around with him as a pitcher? How many longballs have the swirling winds of San Francisco robbed from Bonds? Would everyone be chasing Josh Gibson if baseball had integrated sooner?
8. Ryan Howard
Projected Career Total: 343
Chances of Hitting 500 HR: 16.4%
... 600 HR: 6.9%
... 700 HR: 0.8%
... 755 HR: 0.8%
... 782 HR: 0.8%
... 800 HR: 0.8%
... 900 HR: none
Top Comparables: Travis Hafner, Jim Gentile, Mike Epstein, Cecil Fielder, Mo Vaughn.
Howard is the guy you want up there if your life depends on getting a home run in the next at-bat -- unless the opposing pitcher is a lefty. But he's doomed by the late start to his big league career, and players who are this big and cumbersome (think Vaughn) tend not to age well into their 30s -- the late-starting Howard is already 27. Howard does have one or two comps that are going berserk for him, giving him some trivial chance of challenging the record, but we'd have to be talking about something unprecedented like his averaging 50 home runs per year over his next 10 seasons.
7. Andruw Jones
Projected Career Total: 523
Chances of Hitting 500: 51.5%
... 600 HR: 20.2%
... 700 HR: 3.0%
... 755 HR: 1.2%
... 782 HR: 0.5%
... 800 HR: none
Top Comparables: Griffey Jr., Sosa, Johnny Bench, Dale Murphy, Tom Brunansky.
Jones was a serious sleeper candidate for 755 heading into the year, having cracked the big league lineup as a teenager and staying healthy ever since. But look what's happened now ... not only does he suffer the indignity of drawing Brunansky as a comparable, but we also have the precedent of Murphy, another once-great Braves center fielder who fell off a cliff in his early 30s. That's not to say that Jones, 30, is done, but the record chase does not permit much margin for error.
6. Miguel Cabrera
Projected Career Total: 478
Chances of Hitting 500: 34.2%
... 600 HR: 19.7%
... 700 HR: 13.2%
... 755 HR: 1.8%
... 782 HR: 1.8%
... 800 HR: 1.8%
... 900 HR: none
Top Comparables: Ron Santo, Murray, Alex Rodriguez, Del Ennis, Frank Robinson.
Cabrera's comparables tell an interesting story: He could be an early-peak guy like Ennis, a merely good player like Santo, a slow-and-steady force along the lines of Murray, or an all-time great like A-Rod or Robinson. The longball isn't Cabrera's only strength -- he nearly won the NL batting title last season -- but only about two dozen players in history had more home runs at a comparable age.
5. Manny Ramirez
Projected Career Total: 595
Chances of Hitting 500: 97.1%
... 600 HR: 40.7%
... 700 HR: 6.3%
... 755 HR: 2.1%
... 782 HR: 2.1%
... 800 HR: 1.4%
... 900 HR: none
Top Comparables: Robinson, Sosa, Mantle, Rafael Palmeiro, Gary Sheffield.
I was mildly surprised to see Manny here, but he's always been a player who manages to be both over-hyped and overlooked at the same time. For record-chasing purposes, he would actually be helped by moving out of Boston. Although the Green Monster turns routine flyballs into doubles, the problem is that it does the same thing with potential home runs.
4. Adam Dunn
Projected Career Total: 493
Chances of Hitting 500: 43.2%
... 600 HR: 26.0%
... 700 HR: 7.9%
... 755 HR: 2.6%
... 782 HR: 2.6%
... 800 HR: 1.2%
... 900 HR: none
Top Comparables: Troy Glaus, Mark McGwire, John Mayberry, Jose Canseco, Harmon Killebrew.
Depending on who you talk to, Dunn is either one of the most overrated or the most underrated players in the game. What's more certain is he's surprisingly well positioned to make a run at the record books. Dunn broke into the big leagues relatively early, he's hit for power in good times and bad, and he's stayed healthy, playing in all but seven of his team's games in the past four seasons. One disadvantage is he'll probably be hitting about .210 by the time he hits his early 30s. He could have trouble finding a sympathetic GM at that stage, especially if Billy Beane has moved on to soccer or NASCAR.
The Contenders
3. Prince Fielder
Projected Career Total: 489
Chances of Hitting 500: 45.2%
... 600 HR: 20.4%
... 700 HR: 7.9%
... 755 HR: 6.7%
... 782 HR: 5.6%
... 800 HR: 5.6%
... 900 HR: 5.2%
Top Comparables: Boog Powell, Mayberry, Canseco, Kent Hrbek, Eric Chavez.
He might be the best pure power hitter to come along since McGwire. Prince's comparables aren't terrifically flattering, and there's certainly the chance that by the time he turns 32, he's hung up his spikes and opened up Prince's Peach Pit out in the Miller Park netherworld. Nevertheless, the math is compelling: Fielder projects to become the youngest player ever to hit 50 home runs in a season.
2. Albert Pujols
Projected Career Total: 565
Chances of Hitting 500: 62.0%
... 600 HR: 40.9%
... 700 HR: 14.6%
... 755 HR: 6.9%
... 782 HR: 5.0%
... 800 HR: 5.0%
... 900 HR: 0.3%
Top Comparables: Murray, Orlando Cepeda, Robinson, Greg Luzinski, Jack Clark.
A year ago, A-Rod and Pujols would have been about tied in their likelihood of passing Aaron. But our model is very sensitive to any change in performance levels. It remains unclear whether there's anything wrong with Pujols, who is having a down year by his standards, but projecting him as a 35 HR/year guy instead of a 40 HR/year guy or a 45 HR/year guy makes a huge impact since that difference gets multiplied out over many seasons.
The Front-Runner
1. Alex Rodriguez
Projected Career Total: 731
Chances of Hitting 500: 100%
... 600 HR: 85.6%
... 700 HR: 60.7%
... 755 HR: 46.1%
... 782 HR: 29.6%
... 800 HR: 27.5%
... 900 HR: 9.8%
Top Comparables: Robinson, George Brett, Willie Mays, Bonds, Aaron.
You couldn't do much better in terms of comparables if your goal is to break the all-time record. Nevertheless, we have A-Rod as even-money at best to pass Aaron's mark, and a decided underdog to surpass Bonds' eventual total. What gives? Think about all the things that have to go right for a player to hit 755 (or 782) lifetime home runs. He has to stay healthy. He has to resist the temptation of early retirement, even if he already has several lifetime's worth of money in the bank. He has to avoid any sudden declines in performance. He has to not only stay at the top of his game, but keep the particular skill of power hitting intact.
Rodriguez could "devolve" into being Brett, and that still wouldn't be enough momentum to get him past Bonds and Aaron. It all sounds so easy -- if A-Rod heads into 2008 with about 520 lifetime home runs, then all he has to do is average around 30 home runs per season through age 40 to claim the record from Bonds. But remember when everyone assumed that McGwire -- or Sosa, or Griffey -- would challenge the record?
Remember 10 months ago, when many assumed Rodriguez's best days were behind him? A-Rod's right on pace, but he's too far from the finish line to be conceding any records to him; it's inherently dangerous to be predicting that someone will do something that nobody else in history has done. We might have to deal with this Barry guy for longer than you'd think.
