What it does attempt to measure is the market share currently going to each site from among a diverse audience of web users who probably are not already habitual users of those sites. In other words, it is more a measure of current, sponteneous interest and potential growth among new users than it is an absolute measure of traffic. This list measures how many people seek out sites that have not already been bookmarked or memorized. This is an important factor since one-time use of a site has about a 45 percent market share while habitual site use has only a 13 percent market share, according to other research. These numbers are not perfect, of course.
None ever are.
However, they are based on a sample that is a lot more randomly selected than most panel-based media metrics. Local portal sites are not included because their popularity can be attributed to things other than their news content. Where a content partner providing news is involved, that partner's portion of the site is tracked. This is no different than tracking, say, the nightly news' ratings instead of a network's overall ratings. The list is dominated by sites affiliated with traditional media companies.