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3 External ranking factors
3.1 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.
3.2 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.
 
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