On Tuesday night, Houston Astros ace Roger Clemens, within days of his 43rd birthday, was in vintage form. Over seven dominant innings against the Arizona Diamondbacks, Clemens struck out eight, walked only one, gave up just four hits, and retired 19 of the final 22 batters he faced. An improbable performance from such an old pitcher? Not if you’re Clemens. Indeed, this is what he’s been doing all season.

Clemens is defying a number of expectations this season. You wouldn’t expect a 42-year-old to post the best ERA since Bob Gibson’s 1.12 in 1968. You wouldn’t expect a 42-year-old to more than return value on the dollar on a one-year, $18-million contract (even before accounting for the popular Texan’s influence at the turnstiles). You wouldn’t expect a 42-year-old to turn in one of the greatest single-season pitching performances in the annals of the game. But that’s precisely what he’s doing.

This season, Clemens is a virtual lock to set the ERA record for a pitcher his age. His current mark of 1.45 would easily best Warren Spahn’s ERA of 2.60 in 1963, and in the process would make him the oldest ERA title winner in history. Clemens’s 43rd birthday occurs on Thursday. While, for record-keeping purposes, Clemens will be classified as age 42 for this season, it’s worth noting that his ERA would also easily break the record for 43-year-olds, which is held by none other than the legendary Cy Young (2.53 ERA in 1910).

Records circumscribed by age requirements, however, don’t do justice to Clemens’s work this season. ERA, as a statistic, has its weaknesses. To name only one, it’s too lenient on pitchers following the commission of fielding errors.

If we adjust for park and league, though, ERA can give us a fair picture of how a hurler is performing. It’s after making such adjustments that Clemens’s brilliance this season becomes apparent. That’s because the present era, in historical terms, is an offensive one, and Clemens’s home field, Minute Maid Park, favors the hitter. In other words, he’s putting up that jaw dropping ERA in spite of league-wide trends and the tendencies of his home ballpark.

So if trends hold, Clemens’s ERA of 1.45 will yield a park- and league-adjusted figure (abbreviated as “ERA+”) of 309. That figure is scaled to 100, so Clemens’s ERA+ of 309 means his ERA, after being adjusted for park effects, is 209% better than the league average (a figure derived by dividing the league average ERA by the pitcher’s park-adjusted ERA). Where does that rank all-time? It’s the best ERA+ ever, by a fairly comfortable margin. Here’s the complete top ten:

Clemens is on target to set the single season record for ERA+, which means he would become the first pitcher ever to post a park-adjusted ERA at least 200% better than the league average. Keep in mind his age; the average age of the other nine pitchers in the accompanying chart is 27.

As mentioned above, Clemens’s raw, unadjusted ERA is the lowest since Gibson’s in ’68. However, when Gibson forged his mark, in the “Year of the Pitcher,” the average National League ERA was a mere 2.98. This season, the average National League ERA is 4.29 – 43.9% higher than it was during Gibson’s historic season.

To etch Clemens’s work in sharper relief, bear in mind that Gibson, if he had hoped to equal Clemens’s ERA+ of 309 in 1968, would have needed to post an even more unthinkable ERA of 0.96, which would be the lowest unadjusted ERA since 1900.

In terms of his age group, Clemens has obviously been even more dominant relative to the field. Only six other 42-year-old pitchers in history – Warren Spahn, Nolan Ryan, Jack Quinn, Phil Niekro, Cy Young, and Red Farber – have even topped the 100-point plateau in ERA+ for a season. Spahn’s tally of 124 in 1963 places him second on the list, a full 185 percentage points behind Clemens’s 309 this season.

Here are some other semi-random observations on Clemens’s miraculous season:

* St. Louis’ Chris Carpenter ranks second with an ERA of 2.33, 81 points behind Clemens. So Carpenter is almost closer to Dontrelle Willis, owner of the eighth-best ERA in the league, than he is to Clemens.

* It’s rather astounding that this season Clemens, in the 12 starts that have resulted either in a loss or no-decision, has a 1.54 ERA. That comes to an ERA+ of 290. In other words, Clemens has posted what would be, given a qualifying number of innings, the second-best ERA+ of all-time even in starts that haven’t resulted in a win for him. That’s a dual object lesson on his excellence and the misleading nature of pitcher wins and losses.

* A pitcher is credited with a “quality start” if he pitches at least six innings in a given start and gives up three runs or fewer. In 2005, Clemens has recorded a quality start in all but two of his 22 outings, and in one of those he tossed five scoreless innings against the Cubs. In fact, he’s given up 24 earned runs on the season, which means he has a distant shot at becoming the first qualifying pitcher of the 21st century to record more quality starts than earned runs in a season.

* As mentioned, Clemens’s home digs, Minute Maid Park in Houston, is a hitter’s environment, which means it’s actually hurting the Rocket’s numbers. Focusing on single-year home/road splits is fraught with sample-size hazards, but it’s worth pointing out that Clemens’s 2005 road ERA is 0.31.All season, he’s given up only two runs outside of Houston – one in Coors Field and one against the Cardinals, owners of the NL’s best offense.

All of this drives toward the prevailing point: Clemens is crafting one of the great single-season performances the game has ever seen. Don’t let his modest win total or his team’s relative lack of column inches fool you into thinking otherwise.

Mr. Perry writes for Baseball Prospectus. For more state-of-the-art commentary and information, visit baseballprospectus.com.