We'll need a group of sites to get going.
This software also raises privacy issues, as it relies on tracking a user's behavior on a site, or a group of sites, to determine a consumer's buying preferences.
Another group of sites mixes free content with paid access, using a technique known in the industry as the velvet rope approach.
For those who really want to go into depth on certain aspects of recent events, there is a second group of sites that specialize in original source material.
Google's move expands the online advertising space to the network it has established for text and graphical ads — a group of sites whose number it will not disclose, though it is estimated to be in the hundreds of thousands.
The IknowUK group of sites now lists over 200 B&Bs, small hotels and cottages in the north west, Yorkshire and the Lake District and can identify gay-friendly and non-smoking accommodation and places that cater for pets.
The Rohri Hills, located at the Indus River margins of the desert, contain a group of sites associated with sources of chert, a type of stone that is a principal raw material for making tools and weapons.
As BuzzFeed noted, using data from the BuzzFeed Network, a group of sites that collectively have over 300 million users, Google Reader still sends a considerable amount of traffic to these sites.
"There's a big group of sites arising at the same time," Haas says.
The group of sites is serving over 13 million monthly page views and 1.5 million unique visitors.
A comparison site (or group of sites) is used in order to control for factors other than the road modification.
