To win 100 games in a 162-game schedule, a team needs to be victorious in no less than 61.7% of its games. To lose 100 games, the team can win no more than 38.3% of their games. Currently, no team in either league meets these marks for excellence or desperation, and that compression makes the upcoming trading deadline more complicated, and more important, than usual.

Most deadline deals, in fact most trades, come to nothing, rubbish in and rubbish out. Very few are mishaps on the level of Jay Buhner for Ken Phelps or Heathcliffe Slocumb for Derek Lowe and Jason Varitek. Even fewer trades consummated around July 31 (or in the old days, June 15) truly change a team’s pennant outlook. There are exceptions: The August 12, 1987, trade between the Tigers and the Braves, sending young John Smoltz south in exchange for veteran Doyle Alexander was, in the long-term, an uneven, franchise-destroying trade for Detroit, but in the short-term, Alexander’s 11 stretch-run starts with a 1.53 ERA was decisive in overcoming the Toronto Blue Jays in a tight divisional race. That same year, George Steinbrenner signed off on a July deal which sent three young pitchers — one of whom turned out to be Bob Tewksbury, all-time great control pitcher — to the Chicago Cubs for Steve Trout, in the process turning the Yankees from a probable 95-win team to an 89-win also ran.

It’s much harder to discern the impact of most deadline deals for the acquiring, that is, thecompetitive team. For the team out of contention, giving up a star, it’s clear that they almost never get equivalent value. As for the teams hoping to vault into the postseason, they hope to get doubly lucky, adding the right player (if you replace your B- second baseman with a B second baseman, the change won’t be dramatic) at the right time (capturing a hot streak rather than a cold one). Bonus points if they don’t give up Jeff Bagwell for a playoff-run rental in the process, which the Red Sox did in 1990.

Even if they guess correctly, the short-term benefits are measured in improvements of single wins, not fives or 10s. Fortunately, this year single wins may prove to be decisive. The Red Sox almost certainly have the American League Eastonice, but five other divisions are being contested by teams not quite good enough to seal the deal. In each of those cases, the team that acts first and makes the best deal, stands to win.

The Indians and the Tigers have been swapping the AL Central and wild card leads all season, both having put together strong offenses — in Detroit’s case the best in the business during the first half — and weak bullpens. Both teams have a closer with an ERA over 5.00, which normally wouldn’t be a big deal, as the difference between the best closer and the worst is generally only a few blown saves a season. However, in a race that might be decided in one game, when the Mariners have made the wild card race close enough that the team that falls out of the Central lead might go home before October, those extra losses take on added importance. A bidding war over a sore-armed rental like the Rangers’ Eric Gagne could and probably should erupt between these two clubs, though the loser can comfort itself knowing that adding some padding in the middle of the bullpen — a few less innings going to the Jason Grilli- and Fernando Cabrera-types — could have almost as great a benefit.

In the AL West, the Angels have the best offense in the division, albeit one that has hit fewer home runs than all but three teams (it says something when DH-less NL teams pile up more long balls), and a pitching staff that is almost strong enough. Securing just one starter, or alternatively filing Ervin Santana in some dark corner of the bullpen, would enable the Angels to blow the doors off of the division, despite the charging Mariners. The M’s themselves are overachieving, short of both pitching and offense, and while putting aside H-less DH Jose Vidro might help (something they are rumored to be doing with the promotion of prospect Adam Jones), it is more likely that they will soon fall back.

The Mets will be overturned in the NL East if the Braves can acquire a starting pitcher of any merit, while the Mets will add to their advantage if Omar Minaya can add a bat to the outfield, second base, or catcher. For example, since April, last season’s trade lemon, Shawn Green, has hit .233 AVG/.278 OBA/.369 SLG and needs to be replaced. In the Central, the Brewers are well-situated to continue their run. The Cubs could make yet another run at a middle infielder after dealing Greg Maddux last July 31 and ending up with Cesar Izturis. The Dodgers probably have all the resources they need in-house to press the Padres, while the Pads themselves have all the pitching in the world but need a bat … any bat.

All of these teams will be competing for the same scarce resources, a task made more difficult by the wild card, particularly in the NL, where at least eight teams have hope. Over these next 19 days, the battle for the postseason will be fought off the field, in the offices of the general managers. To the victor will go the spoils. To the loser will go the Trout.

Mr. Goldman writes the Pinstriped Bible for yesnetwork.com and is the author of “Forging Genius,” a biography of Casey Stengel.