The companion web site to the Public Broadcasting System (PBS) network heads a list of the 20 most frequently used nonprofit sites for K-12 classroom instruction—but United States government sites account for more than half the sites on the list.

The data come from N2H2 Inc., maker of the popular Bess filtering software and Searchopolis web portal. N2H2 based its rankings on internet-site usage measured during May through its nationwide network of proxy servers. The company’s figures offer a snapshot of how the internet is being used for instruction in classrooms and school libraries.

N2H2 ranked the sites according to the total time K-12 users spent viewing them. This information gives a clearer picture than total page views because it more directly correlates with each site’s usefulness, said Jim O’Halloran, N2H2’s vice president of marketing. How the sites compare by total page views also is included.