The 2005 World Series was a bummer, at least from the perspective of the Fox network, if not for the Chicago White Sox and their fans. Fox's audience for the four games, all won by the Sox, was down 32 percent compared with last year's four-game Red Sox-Cards Series. Not good.

In fact, the White Sox-Astros Series was the least popular TV Series ever, ducking in below the previous biggest loser, the 2000 Subway Series featuring the Yankees and Mets. Not good at all.

But then, Fox could have seen that coming, couldn't it? Sure it could. Unlike the unpredictable new sitcoms and dramas that roll out each fall, the networks have a pretty good idea in advance how many viewers will watch a big game or championship series. When it comes to drawing a crowd, sports is one of the most predictable things on TV.

All it takes to figure out how a championship series will do in the Nielsens is to know a little history and a few variables.

TV executives know, for example, that the World Series (held during October, when lots of people are watching TV, relatively speaking), will typically perform far better than the NBA championships (held during low-viewing June), but that neither is anywhere near as popular as the NFL playoffs (held during two super-high viewing months, December and January). And almost everything sports-related does better than the woeful NHL playoffs, including U.S. Open tennis and golf.

The next important factor is: Who's playing? Ratings-wise, the best possible matchup is two big-city teams from different regions (preferably from opposite coasts), each with a tradition of success and each with a nationwide fan following. In baseball, that means the Yankees, Red Sox, Braves, Dodgers and (somewhat less so) Cardinals. The Cubs belong on this list, too, even without that whole "tradition of success" thing. So, the ideal World Series matchup might be the Red Sox vs. Cubs or Yankees vs. Dodgers.

Superstar players help, too, but more so in basketball (where the players are constantly on the court) than in baseball (where the stars bat once every three innings, or pitch one game in four). "Barry Bonds is the biggest star in baseball, but the Giants-Angels Series [in 2002] was the [second] lowest-rated until this one," points out adman Bob Dorfman, who writes the Sports Marketers Scouting Report, a guide to athletes and their potential as product endorsers. The all-California Series, he says, dampened interest in the rest of the country.

Traditional rivalries are good, but dynasties are even better. Highly successful teams seem to create their own compelling story line, no matter who their opponent is. In baseball, that means the Yankees. In football, it was the 49ers, Redskins, Cowboys and now the Patriots. The NBA came of age when Magic Johnson's Lakers and Larry Bird's Celtics were trading championships, but TV ratings really took off during the 1990s, when Michael Jordan's Bulls won six championships in eight years.

One more thing: Long is better than short. A competitive six- or seven-game Series builds fan interest more than a quick four and out.

Given all that, it's no surprise that a Chicago vs. Houston World Series flopped with viewers, despite some terrific games. The White Sox are an old franchise from a big city, but they aren't even the most popular team in their home town. The Astros? Little history, not much success and no real fan base outside Houston (Chicago and Houston are top 10 markets, but with fewer than 5 percent of all U.S. TV households, they aren't big enough on their own to carry the whole country). Plus, the most accomplished player on the field, Houston pitching great Roger Clemens, isn't on the field much and isn't exactly widely beloved. Ergo: "Everyone going into this Series was expecting this [the low ratings] to happen," says Dorfman. "I don't think this is surprising anyone."

While Fox undoubtedly would have liked a boffo, seven-game, blow-the-doors-off championship, the disappointment of the Houston-Chicago matchup is cushioned by the fact that there's always next year. The World Series represents just a few games (albeit high-profile ones) in Fox's six-year, $2.5 billion deal with Major League Baseball, which expires after next season.

All the ads that ran during the Series were sold in April and May, says sports consultant Neal Pilson, former president of CBS Sports, so the money went into the bank long ago. Even if the Series's ratings fall below the levels that Fox projected for advertisers back then, the network won't give back any money. It will instead offer "make goods" -- free ads -- on programs it airs months from now. Often, says Pilson, this is time the network couldn't sell, or couldn't sell at very good rates. "For a TV network, this is business as usual," he says.

Which is why you're not hearing too much whining about lousy ratings from the minions at Rupert Murdoch's network. "There are a lot of positive things happening with baseball,'' Fox Sports President Ed Goren recently told Advertising Age magazine. "We have a 10-year history. I don't see one year [of ratings] making a difference."

Astros fans in League City, Tex., watch their favorite team under the stars. Nationwide, they didn't have much company.In Roger Clemens, the '05 World Series had just one mega-star. Alas, he couldn't pitch every game.