Over the last few years, parents have been watching their teens' Internet usage more closely, with more than half of them using software filters to keep pornography and other offensive content from their children, a research firm says.

But exactly how much impact parents' increased vigilance has on their children's online behavior is unclear, according to a November 2004 survey of Internet-connected families by the Pew Internet and American Life Project.

Fully 54 percent of families surveyed used filters in their home computers, compared with 41 percent in December 2000, Pew said in the study released this week. In terms of numbers, that's 12 million families today using programs like Net Nanny and CyberPatrol from 7 million in 2000, or a 65 percent increase.

But about 8 out of 10 parents and teens agreed that the latter aren't careful enough when giving out personal information online, and more than 6 out of 10 in both groups said teens do things online that they wouldn't want their parents to know about, the research firm said.

Among youth between the ages of 12 and 17, 87 percent use the Internet, or about 21 million, Pew said in the study released this week. Among those using the Internet, 19 million, or 87 percent, have web access at home, while the rest use computers in such places as schools, community centers, churches, friends' homes or cyber cafes.

In addition to technology, parents are using other means to control their children's online activities. Nearly three-quarters of the teens surveyed said their home computer is located in a public place, and 64 percent of parents said they set rules about their children's time online, Pew said.

Nevertheless, there's a huge gap in perception between parents and teens on how much monitoring is taking place. More than 6 out 10 parents said they checked their children's surfing habits after they have gone online, but only 33 percent of teens said they believed their parents monitored their online activity.