Both the Cardinals and Astros have 30 wins each and the best record in their respective leagues. But which team is more likely to sustain its hot start?

As major league franchises go, the list of things the Cardinals and Astros have in common is quite short. St. Louis is a perennial contender which has won more modern World Series (11) than any team other than the Yankees. Houston, meanwhile, is a perennial doormat which has never won a Fall Classic and was swept in its only World Series appearance a decade ago. In the last four seasons alone, the Cardinals have won two pennants and a world championship, while the Astros have suffered three last-place finishes and averaged 104 losses per season.

As of Wednesday, however, both teams currently boast 30 wins and the top two records in baseball more than a quarter of the way through the season. And while there is a temptation to see the Cardinals’ performance as real and the Astros’ as a fluke, a closer look at their performances lends little support to that perception.

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The obvious place to start is run differential. The Cardinals have a big lead there, outscoring their opponents by 58 runs on the season to the Astros’ 29. That lends significant support to the idea that the Cardinals are the better team, but it doesn’t suggest the Astros are a fluke. Indeed, the only team in the American League with a better run differential than Houston is the Royals at +48. The Cardinals’ Pythagorean record (derived from run differential) is a dead match for their actual 30–16 record, but the Astros’ Pythagorean mark of 27–20 only falls three wins shy of their actual 30–17. Look a little closer and you can find the source of that difference: Houston's record in one-run games. The Astros are 11–5 in one-run contests, a record which trends toward .500 even for good teams. Houston has thus out-played expectations by three wins, an exact match for the degree to which the team's actual record exceeds its Pythagorean.

Don’t write off that three-win difference as a fluke just yet, however. There is some actual performance behind the Astros’ success in one-run games. Buoyed by the additions of veteran free agents Luke Gregerson, who is now Houston’s closer, and Pat Neshek, Houston has the second-best bullpen ERA in baseball with a 2.10 mark, behind only the Royals’ 1.86. Good relief pitching is essential to success in close games, and while both the quality of a bullpen and a team’s success in one-run games can be difficult to sustain from year to year, it’s not uncommon for that combination to result in a surprise single-season performance. For proof, look at the Orioles in both 2012 and '14.

Things look even better for Houston if we go beyond simple Pythagorean record to third-order record, which calculates expected runs scored and allowed from the elements of run scoring (hits, walks, etc.) then extrapolates a team’s expected record. There we find that the underlying performances of the Astros and Cardinals have indeed been of comparable quality this season: Third-order record sees St. Louis as a 27–19 team and the Astros as a 28–19 team, a half-win better.

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One area in which the Cardinals appear to be succeeding beyond the actual quality of their performance is run prevention. St. Louis has been the stingiest team in the majors thus far this year, allowing just 3.04 runs per game and posting a staff ERA of 2.79. But the team's Fielding Independent Pitching is just 3.38. The Astros, meanwhile, have a 3.53 staff ERA, which is right in line with their FIP of 3.55. This would appear to tell us that the Cardinals have benefited more from the quality of their fielders than the Astros. But Houston actually has better raw and park-adjusted defensive efficiencies than St. Louis, and the Astros' primary catcher, Jason Castro, has graded out as a better pitch framer than Cardinals stalwart Yadier Molina this season, per Baseball Prospectus’s numbers. Given the significant gap between St. Louis’ ERA and FIP, it would seem some correction is coming regarding run prevention, which should narrow the run differential and bring the Cardinals closer in line with Houston.

Another point in the Astros' favor is strength of schedule. Using the third-order records of their opponents, the Cardinals have thus far had an easier schedule, with their opponents combining for a weighted third-order record of .491 to Houston's .502. St. Louis has played 10 games against teams with a third-order record below .400—six against the Brewers (.352) and four against the Phillies (.381)—and gone 7–3 in those games, while the Astros have not played a team with a third-order record worse than the Padres’ .437. Unlike that run prevention disparity, however, the schedule mismatch is not going to change. There are currently just three teams with third-order records below .400: the Brewers, Phillies and White Sox (.386). From among that trio, the Astros have only six games against Chicago remaining on their schedule. The Cardinals, meanwhile, have 20 games remaining against those three teams, half of them coming against Milwaukee.

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That softer schedule may make it easier for the Cardinals to sustain their hot start, but there are reasons to see the immediate future as bright for the Astros, as well. Specifically, there's the arrival of top prospect Carlos Correa after the Super Two deadline passes in June. The top pick in the 2012 draft, Correa, a 20-year-old shortstop, was a top-four prospect coming into the season and was promoted to Triple A earlier this month after absolutely destroying Double A in his debut at that level (.385/.459/.726 with seven home runs in 29 games). He is off to a strong start in Triple A, as well, hitting .283/.353/.500 in 68 plate appearances. That's good news for Houston, which is lacking at shortstop after losing Jed Lowrie to a torn thumb ligament in April, leaving the position to a combination of Marwin Gonzalez and Jonathan Villar. Calling up Correa could upgrade the position on both sides of the ball.

Houston has already improved its rotation with the aggressive promotion of 21-year-old righty Lance McCullers, another 2012 first-round pick, who has impressed in two big league starts after getting the call straight from Double A. The Astros could do the same for their lineup by recalling first base prospect Jon Singleton (.277/.383/.614 with 14 homers in 44 games at Triple A) to take at-bats away from Evan Gattis and Chris Carter (a combined .190/.263/.384 with 104 strikeouts in 350 PA on the season). Remember, service time is a non-issue with Singleton, who signed a five-year, $10 million contract before his major league debut last year and will earn $2 million this year regardless of where he plays. Great as the Cardinals’ farm system has been in recent years, St. Louis lacks the potential for comparable in-house upgrades.

None of this is to suggest that the Cardinals aren’t for real. But if they are legitimate contenders, then so far, so are the 2015 Houston Astros.