To DVD or not to DVD? That is the question facing studio executives deciding how much of what's in their television library to put on the market.

It's often not as simple as giving a green light to a hit series.

"TV has sort of taken the science out of the business," said Marc Rashba, vice president of catalog marketing for Sony Pictures Home Entertainment. "Theatrical business is primarily driven by: If it opened big as a movie, you have a good idea there's [interest]. ... But with TV, we've learned that ratings don't necessarily translate to success on DVD at all."

"There's no magic formula," agreed Todd Rowan, vice president of marketing for Fox Home Entertainment. "There's a whole confluence of things you look at."

Those include the show's ratings, the genre, the demographic the show most appeals to, what market research says consumers are most interested in and what retail outlets are most excited about selling.

Despite the importance of Nielsen ratings in whether or not a TV show continues in production and remains on the air, it's not the best indicator of DVD sales. Rowan said genre is the most reliable predictor. Shows in the sci-fi/fantasy genre often do best on DVD.

"If it's younger-skewing, if it's contemporary and it has a bit of an edge to it, maybe sci-fi, and it has a cult-like following, it will do better," Rowan said.

Rosemary Markson, executive director of TV marketing for Warner Home Video, agreed.

"It's the collector out there who loves these shows, and that's the beauty of what's driving demand for the show," she said.

Rowan pointed to his company's success with "Firefly," a short-lived sci-fi series from "Buffy the Vampire Slayer" executive producer Joss Whedon. The TV show lasted less than a season, but the boxed set sold more than 200,000 units and a feature film based on the series will hit theaters in September.

"It exceeded our expectations by almost double," Rowan said.

On the other hand, the first season release of a onetime hit like "NYPD Blue" generated "just OK" sales for Fox, Rowan said, although subsequent season boxed sets are in the works.

If your favorite series hasn't been released on DVD, it's not necessarily a matter of taste, or even demand.

Although more than 2,400 television series are available on DVD, there are still plenty of programs that aren't. Sometimes, the decision is made based on demographics or a perceived lack of demand. But often, high-profile series are delayed due to either complicated rights issues or a prohibitive cost involving the show's music.

"A lot of those original show agreements were silent on the [home video] issue, but they were co-productions between two studios," Rashba said. Some of those companies no longer exist. Or they may not have a great relationship.

A DVD release might hinge on tracking down a series' originators or dealing with their estates. "It's really difficult when you get into that to find out who really does have the rights," Rashba said. "Ultimately the studios or whoever worked on the shows all have to get back together and talk about a new deal to cover that."

The biggest hindrance to many releases is clearing music rights.

"The cost of musical compositions -- not the performers, the writers, the directors, just the songs they sing -- can add up to more than we'd ever see in a royalty from a DVD sale," said Paul Brownstein, a producer of DVD extras.

This is especially true of older shows because contracts and union rules simply did not account for home video because it didn't yet exist.

But it's still an issue today. For the DVD release of the 2004 series "Wonderfalls," executive producer Todd Holland said $600,000 worth of music used in the TV show was reduced to $200,000 in music.

"All series do this because they don't want to pay for the original music," Holland said. "Home video is like a whole different licensing fee from the rerun licensing fee."

Holland said "Wonderfalls" retained what he considered the most important popular background music, often heard in the background of scenes set in the show's bar hangout, but replaced other songs with less costly tracks.

"Some of those incidental songs have changed," he said, "so you're not playing Gwen Stefani -- you're playing somebody who sounds like Gwen Stefani."

Music rights are a big stumbling block for Fox Home Entertainment's desire to release "Ally McBeal," a series heavy with music. Singer Vonda Shepard performed classic (re: expensive-to-license) tunes every week.

"I would love to put out 'Ally McBeal,' but I can't," Rowan said. "The music in the show is such an integral part of that show, you don't swap that out. It's part of the viewing experience of the show. I can't say never, but it's not in our release plans anytime soon."