What is an authoritative link source?
In my opinion, a good answer to this question can be found by reviewing what a Google engineer has written on this topic. Krishna Bharat (Google Engineer) and George A. Mihaila wrote a paper called “Hilltop: A Search Engine based on Expert Documents” in which they
“..propose a novel ranking scheme for broad queries that places the most authoritative pages on the query topic at the top of the ranking. Our algorithm operates on a special index of “expert documents.” These are a subset of the pages on the WWW identified as directories of links to non-affiliated sources on specific topics.”
The gist of the Hilltop paper is that a site that has many links from category specific hub pages that match the user query will be more relevant. This is really not much different than what consider as hub and authority pages today. However one thing that often gets overlooked is the part about how the hub pages (‘expert documents’ as they’re referenced in the paper) are scored. The only parts of the hub pages that get indexed, and consequently are used to compute relevancy, are the page titles, the headings, and the anchor text:
“..we only index text contained within “key phrases” of the expert. A key phrase is a piece of text that qualifies one or more URLs in the page. Every key phrase has a scope within the document text. URLs located within the scope of a phrase are said to be “qualified” by it. For example, the title, headings (e.g., text within a pair of H1 & /H1 tags and anchor text within the expert page are considered key phrases. The title has a scope that qualifies all URLs in the document. A heading’s scope qualifies all URLs until the next heading of the same or greater importance. An anchor’s scope only extends over the URL it is associated with.”
I believe this is one of the primary ways that Google determines the relevancy of a link. The amount of relevance given to a link can be based on how the URL is ‘qualified’. If the page title, headings, and anchor text are all relevant (but make sure they’re not identical to avoid tripping over optimization filters) to the user query, there is a very high percentage chance that the linked URL is also relevant, more so than if some of those relevance signals were not there.
How Can I Use This?
This is something that can be utilized when building links. Obviously, this further validates (if you believe Google has implemented elements of the Hilltop algorithm, which I think they have) the enormous impact that a presell page can have on rankings. Other ways to use this information are if you are syndicating content that has links to some of your other pages, try to use variations of your keywords in “key phrases” throughout the page as often as possible without sounding unnatural. If you are buying links, shame on you! But while you’re at it, see if you can pay extra to have the page title changed, or see if you can provide the text surrounding the link instead of having them write it for you, and see if you can include an h2 tag or something above your link with some keywords.
Think Like a Search Engine
As a link builder, I think it’s important to have a general idea of what the search engines are trying to achieve. The more you learn will allow you to get more benefit out of your links. By having a general idea of what the engines are trying to accomplish plus what data they have access to (just about everything) and may use to compute relevancy, you can make educated guesses as to what may work well and what will not, and how to utilize keywords throughout the page.
The most direct way to cultivate this train of thought in my opinion is to read research papers and patent applications. I admittedly suck at math but I am still able to understand the general idea of most research papers and patent applications. Some of the other papers are more complex, but I don’t think I’ve come across one where I couldn’t grasp the overall concept. And if I can do it, anybody can. Bill Slawski’s blog is a great resource for this type of info and should be read by anyone who is interested in learning more of what the search engines are trying to accomplish.