NEW YORK — Most parents of teenagers who go online say they set time limits on the kids' Internet activity, according to a study released Thursday. They also try to monitor it, in part by placing computers in common areas.

Parents also don't shun technical tools.

Slightly more than half of parents with online teens — 54 percent — have filtering software installed on home computers, up from 41 percent in 2000, the Pew Internet and American Life Project study found.

Another of the survey's findings: 13 percent of American teens do not use the Internet at all.

Parents who are themselves online are more likely to set rules about when or for how long their kids can log on at home. Overall, nearly two-thirds of parents with online teenagers say they have such rules.

Nearly three-fifths keep the computer in an open area, like the living room.

"You set rules about television watching, cleaning up after yourselves, doing the dishes," said Amanda Lenhart, a Pew research specialist and author of the study. "Doing it for online safety is an easy next step for parents to take."

Lenhart cautioned, however, that time limits can result from family members competing to use a single computer rather than an effort to restrict kids' usage.

She added that kids who have such rules at home are also more likely to access the Internet from school, possibly negating the impact of any rules.

Mothers are more likely than fathers to install filters. And younger parents and those who go online at least once a day are also more likely to use employ filters to try to prevent their children from viewing pornography and other material they deem objectionable.

In addition, 62 percent of parents of online teens say they check the sites their children visit, though only a third of teens who use the Internet at home believe their parents do so. And any reasonably tech-savvy teen knows how to erase their online footsteps.

The perception gap is significant, said Parry Aftab, an Internet safety expert who runs workshops for parents and children.

Aftab questions how strictly parents are really controlling their children's time online. Few, she said, use software to enforce those limits.

And while the Pew finding on open access is comparable with those from other surveys and anecdotal evidence, Aftab notes that many kids are now getting online through cell phones, video game consoles and other gadgets that may not be centrally located.

Lenhart agrees that time limits and computer location won't replace the need to educate kids about Internet use and safety.