The races to the AL wild card and Eastern division playoff slots are in full swing, with the Yankees, Red Sox, Twins, and White Sox in the best positions to advance. The four teams are all knotted up around 70 wins with 45 left to play, and all four rank among baseball’s top offensive ballclubs. However, the distinction between these teams’ rotations — and more importantly, who’s in those rotations during the next seven weeks — might be all of the difference between an October trip to destiny versus a golf outing in Palm Springs.

To evaluate rotation performance, you can rely on several different metrics, but let’s look at one old standby (ERA), one set of data that tells us pretty concretely how many winnable ballgames a guy has pitched (quality starts), and a more advanced stat, Support-Neutral Lineup-Adjusted Value Added, or SNLVA.

Quality starts aren’t all that complicated: six innings or more, three runs allowed or less. While traditionalists might complain that three runs allowed in six innings wasn’t very good in their day, they’re forgetting that it actually was — the 60’s didn’t last forever, and Sandy Koufax didn’t pitch for everybody.The quality start is particularly important to teams with quality lineups, as these four contenders boast, because the more often a pitcher gives you that plain old quality start, the more likely you’ve busted out offensively and built a lead you can hold.

We’re also adding Blown Quality Starts to the mix. That’s a game where a starter delivered a quality start through the first six frames, only to lose it by allowing that fourth run in the seventh or later. There can be all sorts of reasons for a Blown Quality Start: maybe the pen was overworked the night before, or maybe the manager was certain he’d be able to get another inning out of his horse. Regardless, the starting pitcher did his job, so we’ll note when he did. If you’re getting quality starts from a pitcher significantly more than half the time, you’ve got somebody you can beat people with.

The more complicated number is SNLVA. Here, the pitcher’s performance is evaluated not simply on the events that took place, but is interpreted on the basis of who he was pitching against. This is a basic adjustment for the fact that not all pitchers face the same opponents the same number of times.

Not everybody gets to face the Royals the same number of times, after all, and that isn’t quite fair. SNLVA levels the field to give us a better look at who’s especially valuable. Numbers larger than 1.0 are particularly good; anything higher than 2.0 is remarkable.

The key consideration here isn’t how good or bad the pitching staff has been so far — the Yankees don’t have to keep trying their luck with Aaron Small or Shawn Chacon, after all, while the Twins are going to have to make do until they find out whether Francisco Liriano will be able to help them again this season. If you looked at a rotation’s cumulative performance, you lose the trees for the forest.

Boston has a classic problem: They have an extensive blend of star players (like Schilling and Beckett), and in theory, they have inning eaters like Tim Wakefield, David Wells, and Matt Clement.But those three have been hurt, and only Wakefield has been effective. Veterans like Jason Johnson (-1.8 SNLVA, and only six quality starts out of 19) haven’t helped, while the much-ballyhooed Beckett has been particularly disappointing. If there’s a danger sign for this staff, it’s that manager Terry Francona might be going to the whip too soon: after getting 13 quality starts from Schilling in his first 17, Schilling’s blown two and logged only three in his last eight. With nobody besides Schilling pitching well, losing him to exhaustion would make the Bombers’ lives that much easier.

The Sox’ rotation success is built on depth and durability, with only one starter really overawing the league. Manager Ozzie Guillen believes in pushing his starters as deep into ballgames as he can get them, which suits horses like Garcia and Buehrle fine. The Sox lead the majors in starter innings pitched, with 750.2. It might not work quite so well with Vazquez, who’s beginning to look like a 75-pitch Cinderella: before 75, he’s solid, but on pitch no. 76 and every pitch thereafter, he’s getting tattooed at a .321 clip, and hitters are slugging .579 off of him. If Guillen can stop trying to make a silk purse into a sow’s ear, he’d have a nice starter, albeit a limited one.

Where Santana and Liriano gave the team an unparalleled pair of starters, they’re now down to one. Top-rated rookie Matt Garza might get high marks for his stuff and for his lightning-fast promotions since being picked in the first round of last year’s draft out of Fresno State, but last week’s major-league debut was not Liriano-level good. Even with Silva bouncing back from throwing only one quality start in his first eight, the Twins’ playoff shot has been desperately wounded, being down a man they can’t afford to lose.

Like the White Sox, the Yankees get reliability top to bottom, especially now that they’ve added Lidle to the fold. Unlike Guillen’s charges, Joe Torre’s staffers have had to be worked more carefully, throwing almost 100 fewer innings (661). Mike Mussina has cooled off from a redhot start: after beginning with 11 quality starts in his first 12, he’s recorded a more pedestrian six in his last 13. Hopefully, the lighter workload thus far will make it easier for Torre to count on his fivesome down the stretch. The real hope is that the Yankees get a shadow of the Big Unit’s former dominance in time to give them another stopper in September.

Ms. Kahrl is writer for Baseball Prospectus. For more state-of-the-art analysis, visit baseballprospectus.com.