A few years ago, the TV networks had good reason to fear digital video recorders, with their capacity to zip through ads.

But as it turns out, DVRs are proving a boon for the networks in one important way. They're creating more opportunities for viewers to watch television, and viewers are doing just that--watching more TV, and more network TV at peak primetime hours.

DVRs, which are now in 31 percent of homes, up from virtually nothing just three years ago, are being used primarily to record network primetime shows, particularly on nights when the networks have hit shows competing against one another, according to a new study by Nielsen.

Nielsen found that DVR users are watching more episodes of high-rated shows each month than viewers without DVRs.

People who recorded programs and watched hit shows up to a week after their original broadcast saw an average 2.5 episodes of ABC’s “Grey’s Anatomy,” for example, compared to people without DVRs, who were found to have watched 2.1 episodes.

“The main reason people like DVRs is that they can watch what they want when they want,” says Bruce Leichtman, president and principal analyst at Leichtman Research Group in Durham, N.H. LRG regularly tracks how DVR users watch TV.

“The second thing is that it’s an easy way to record. People forget that. It’s just an easy way to record TV shows.”

DVR viewing actually spikes in October and November, early into the new broadcast season when networks are loading up on original episodes.

During those two months last fall, program playback in DVR homes generated a 10.3 rating among adults 18-49, most of it for network programs, compared to about a 6 rating during the summer and roughly an 8 rating for much of the rest of the year.

Nielsen also found that most DVR playback among 18-49s occurs during primetime, peaking at 9 p.m., when almost 12 percent of all playback takes place.

The vast majority of playback, nearly 100 percent, occurs within a week of a show’s original broadcast, but even more important to the networks, about 90 percent of playback takes place within three days.

That's a key figure in media buying. Buyers and sellers negotiate based on C3 ratings--commercial ratings for live viewing and three days of DVR playback.

As for ad skipping, it appears not to be nearly the threat the networks once thought.

Researchers report that fast-forwarding through commercials doesn’t vary much between heavy users of DVRs and light users, suggesting the amount of commercial skipping going on now probably won’t change much as DVR penetration increases.

In November last year, the heaviest DVR users watched commercials about 44 percent of the time. The lightest users watched commercials about 45 percent of the time.

In his study, Leichtman found that 90 percent of DVR users say they sometimes fast-forward through commercials.

But a large percentage of people without DVRs also avoid commercials.

About 64 percent of people without a DVR in LRG’s research say they don’t watch commercials, either by flipping channels or leaving the room.

“People are skipping commercials, absolutely,” says Leichtman. “But we operate under the assumption that DVRs invented commercial slipping. No, the dial invented commercial skipping and remote controls made it easier.”