External ranking factors I

Why inbound links to sites are taken into account

As you can see from the previous section, many factors influencing the ranking process are under the control of webmasters. If these were the only factors then it would be impossible for search engines to distinguish between a genuine high-quality document and a page created specifically to achieve high search ranking but containing no useful information. For this reason, an analysis of inbound links to the page being evaluated is one of the key factors in page ranking. This is the only factor that is not controlled by the site owner. It makes sense to assume that interesting sites will have more inbound links. This is because owners of other sites on the Internet will tend to have published links to a site if they think it is a worthwhile resource. The search engine will use this inbound link criterion in its evaluation of document significance. Therefore, two main factors influence how pages are stored by the search engine and sorted for display in search results: - Relevance, as described in the previous section on internal ranking factors. - Number and quality of inbound links, also known as link citation, link popularity or citation index. This will be described in the next section.

Link importance (citation index, link popularity)

You can easily see that simply counting the number of inbound links does not give us enough information to evaluate a site. It is obvious that a link from www.microsoft.com should mean much more than a link from some homepage like www.hostingcompany.com/~myhomepage.html. You have to take into account link importance as well as number of links. Search engines use the notion of citation index to evaluate the number and quality of inbound links to a site. Citation index is a numeric estimate of the popularity of a resource expressed as an absolute value representing page importance. Each search engine uses its own algorithms to estimate a page citation index. As a rule, these values are not published. As well as the absolute citation index value, a scaled citation index is sometimes used. This relative value indicates the popularity of a page relative to the popularity of other pages on the Internet. You will find a detailed description of citation indexes and the algorithms used for their estimation in the next sections.

Link text (anchor text)

The link text of any inbound site link is vitally important in search result ranking. The anchor (or link) text is the text between the HTML tags «A» and «/A» and is displayed as the text that you click in a browser to go to a new page. If the link text contains appropriate keywords, the search engine regards it as an additional and highly significant recommendation that the site actually contains valuable information relevant to the search query.