Projecting career stats is an inexact science

Here at ESPN, we're pretty sure we've put Pedro Gomez's face on your TV screens enough times to alert you to the fact that some guy named Barry Bonds is about to hit a fairly momentous home run.

Well, it's time to change the subject (at least for the next minute and a half).

We might want to mention that another man in baseball just hit a home run that, in its own way, could be considered almost as momentous.

We recognize that a 500th home run doesn't carry the same magnitude as a 755th home run. But if the man hitting it is named Alex Rodriguez, it becomes a 500th home run of vastly greater significance than just about any of the 21 previous 500th home runs.

Alex Rodriguez hit that 500th home run Saturday, eight days after his 32nd birthday. Let's try to give you some perspective on what that means.

• That's two years and five months younger than Hank Aaron was when he hit No. 500.

• It's two years and four months younger than Willie Mays was when he hit No. 500.

• It's three years and 10 months younger than Mark McGwire was when he hit No. 500.

• And it's four years and eight months younger than Bonds was when he hit No. 500.

Or think about this another way. A-Rod could blow out his knee, or have Tommy John surgery, or find some other reason not to play another game until Labor Day of 2009 -- and still have more home runs than any of those guys had at the same age.

But since we haven't seen Dr. James Andrews lurking around his locker lately, we're assuming that won't be happening. So as Barry prepares his 756th-homer speech, Alex Rodriguez just keeps on making those home run trots.

Another 256 of them, and he and Barry can team up for their own ESPN Classic Home Run History marathon.

But this is where we need to stop this parade before we start decorating all the floats. We're amazed by all the people who pretty much assume that it's now only a matter of time before A-Rod reclaims the record from that scoundrel, Barry. Well, here's our advice to all of those people:

Heed the powerful lessons of history. Pay attention to the almost never-ending message the sports world delivers to us every single day. Which is:

You can't assume anything.

Has A-Rod put himself in perfect position to blow away this record? Of course. He's younger than guys like Mark DeRosa and Gary Matthews Jr., and he's already at 500 homers. But that doesn't necessarily mean he's a better bet to break Bonds' record than he is to make more money next year than the entire work force down at 7-11.

For one thing, we can't be sure where Bonds' own total is going to come to rest. For argument's sake, let's say it's 775. Could be more. Could be fewer. But let's just use 775 as our guidepost.

That means A-Rod would need another 275 homers to catch him. And for a guy like A-Rod, hitting 275 home runs probably sounds like it would be an easier job than editing his wife's T-shirt collection.

But hold on. Let us remind you that 275 home runs is still a mess of homers -- even in this day and age. It's more than Roger Maris, Steve Garvey, Kirk Gibson, Brooks Robinson, George Bell and about 400,000 other men hit in their whole careers, as a matter of fact.

And the other thing we all need to remind ourselves -- always -- is that we have no idea (none!) how many seasons, how many at-bats, how many opportunities Alex Rodriguez will get, over the rest of his career, to break that record.

Maybe he'll keep cranking out big home run years into his late 30s and early 40s, like Aaron and Bonds. But the odds say he won't.

Mays and Mickey Mantle started sliding precipitously at age 36. McGwire was done at 37.

Jimmie Foxx hit 464 runs before he turned 32. But he was all but washed up by age 33.

And then there's Junior Griffey.

Could Griffey's career, before age 32, possibly have been more A-Rodesque? Like A-Rod, Griffey was the No. 1 pick in the draft. Like A-Rod, he started his career as a teenager in Seattle.

Griffey hit 460 home runs before his 32nd birthday (in 2001). And he would have been extremely close to A-Rod's total if he hadn't suffered what looked, at the time, like a freak torn-hamstring injury in spring training of 2001. Other than an earlier broken wrist, from crashing into a fence, it was the only serious injury of Griffey's career to that point.

But then, just as we were all about halfway through our big Why-Junior-Will-Pass-Hank term papers, you know what happened:

Griffey turned into a walking HMO ad.

Maybe that will happen to A-Rod. Maybe it won't. But not even baseballpsychic.com can tell you that answer. That part is up to the baseball gods.

So all we can do is look to the past for guidance. And that's what we did. We studied what happens to the great sluggers after they blow out the candles on their 32nd birthday cake. The results are fascinating:

• Of the top 10 names on the all-time homer list, seven of them had a season that produced the best home run ratios of their careers after they turned 32. So that's good.

• But only three of them (Aaron, Bonds and Rafael Palmeiro) had that season in their late 30s. So that's bad.

• And if you think it's so easy to hit 275 home runs at this stage of a career, consider this: Only six players in history have ever done it -- Bonds, Aaron, Ruth, Mays, McGwire and Palmeiro. And there are, shall we say, serious performance-enhancement questions about half of that group.

• You may also notice that every one of those guys was an outfielder, a first baseman or a DH. None was a third baseman. And none was a shortstop -- which could be relevant, since A-Rod has made noises about returning to his old position if he leaves the Yankees. The only third baseman who ever got within 50 homers of hitting 276 after turning 32 was Mike Schmidt (237). No shortstop ever hit more than 100 homers after turning 32. (Eddie Joost hit 100 on the nose.) And Cal Ripken hit 162 while dividing his time between short and third.

So what does all this say about Rodriguez's chances of passing Barry?

One man who is bullish on those chances is David Vincent, the legendary SABR home run historian. Despite the lack of precedent for any third baseman or shortstop hitting this many home runs at this stage of his career, Vincent thinks A-Rod's position actually gives him an advantage over someone like Griffey.

"A-Rod plays a position now that will take less of a toll on his body than center field," Vincent said. "He does not routinely run into walls or dive for balls like Junior. I think he will be healthier, which could enable him to play longer at a top level."

A-Rod has averaged 46 homers a year over the 10 full seasons in his career (not even counting this year). So Vincent finds it easy to envision him averaging 35 a year for the next eight seasons -- which would get him to 780 or so.

But that always seems easy to envision when a player is in the prime of his career. Our question is whether that's a mathematically- or historically-safe assumption.

For that answer, we turned to Dan Heisman, a sabermetrician who for years has used variations on Bill James' Favorite Toy formula to analyze active players' odds of achieving certain milestones. Here is what he found:

• Using projections for this year's totals, he calculated that A-Rod has a 48 percent chance of hitting 775 homers (or 52 percent of hitting 756).

• The only other hitters who even had better than a 15 percent chance at 775 were Albert Pujols (26 percent), Ryan Howard (18 percent) and Adam Dunn (also 18).

So obviously, A-Rod has put himself in by far the best position of any other active player. But even that 48 percent calculation tells you he's still a stronger bet not to catch Bonds than to catch him.

And remember, as recently as 2000, when Griffey had 438 homers at age 30, he was rated as having a 42 percent chance to hit 756, and a 38 percent chance to hit 775. But by the time Griffey arrived at No. 500 -- four years and an assortment of major injuries later -- his chances had decreased slightly.

Like to zero.

So just for fun, we asked Heisman if he could also calculate what the chances looked like for the next youngest 500-homer men -- Aaron, Ruth, Mays, Sammy Sosa and Foxx -- at the time they hit their 500th.

Sosa turned out to be the guy who topped them all, at 39 percent (and a 44 percent chance of reaching 756). Ruth was next at 29 percent (35 percent for 756), Foxx at 24 (29 for 756) and Mays at 22 (27 for 756).

But wait. What about the guy who wound up as the actual record-holder, Aaron? You'll be either stunned or amused to learn that he projected to a zero percent chance back then at either 756 or 775 (because he'd slumped to only 29 homers the year he hit his 500th).

And that tells you all you need to know about the dangers of getting into the projection business -- or the assumption business -- on stuff like this. It's a reminder that the chances often look great of all kinds of records falling -- until they don't.

So as A-Rod celebrates No. 500, feel free to applaud the feat and savor the moment. Then it'll be time to kick off this very argument at a tavern near you.

Why do we have a feeling we'll still be kicking at it in the year 2013?

Jayson Stark is a senior writer for ESPN.com. His new book, "The Stark Truth: The Most Overrated and Underrated Players in Baseball History," has been published by Triumph Books and is now available in bookstores. Click here to order a copy.