Google's PageRank is another criteria widely used. Check the links. 

http://en.wikipedia.org/wiki/PageRank

https://support.google.com/toolbar/answer/79837?hl=en

http://www.webmasterworld.com/google/4657878.htm

Hi Jose

Thank You for answer

Hello Jan,

I have been compiling a selection of "Google friendly" website tools for several years, included is a selection of ranking tools and devices to improve your SEO, the list continues to grow.

http://www.apexbusinesssupport.com/articles/great-tools-for-use-on-your-website/

I use these tools myself to monitor the websites that I manage and develop - I also have a selection of tip sheets to help with other marketing

http://www.apexbusinesssupport.com/articles/tag/Top%20Ten%20Tips/

- hope you find it as useful as I do, feel free to make contact if you need further help.

Thanks,

PaulAtApex

http://www.apexbusinesssupport.com/articles/great-tools-for-use-on-your-website/

http://www.apexbusinesssupport.com/articles/tag/Top%20Ten%20Tips/

http://www.apexbusinesssupport.com/articles/tag/Newsletter/

@José Eduardo Jorge: Google's PageRank is another criteria widely used.

Google's page rank algorithm is given in

A. Marcus, On the mathematics of ranking universities and scientific products:

http://conference.ubbcluj.ro/competitiveness/files/marcus_math-of-ranking.pdf

Basicallyl, Pagerank bases its ranking of a web page based on a combination of the number of hyperlinks that point to the page and the  status of the pages the hyperlinks originate from (p. 7).

A personalized form of Pagerank is given in

P. Sarkar, Tractable Algorithms for Proximity Search on Large Graphs, Ph.D. Thesis, Carnegie Mellon University, 2010:

http://www.cs.cmu.edu/~psarkar/thesis/sarkar_thesis.pdf

See page 13 on Personalized Pagerank.    The is an interesting study, well-worth pursuing.

 Very interesting stuff, James. We also know that Page Rank is not the only criterion Google use to decide the position of a given document in its search results page (SERPs). Many webmasters says that useful content is what matters, but academic content --including the fact that most of our articles are pdfs, not html-- seems to present an additional challenge. We have the same documents in several academic networks, plus the original journal (maybe an open journal) and university repository, including links in different directions. I'm still examining downloads and SERPs  and trying to figure out the best strategy. It would be interesting to discuss this. 

An interesting question. It is indeed interesting, for example, an algorithm according to which certain search results appear in a Google search engine based on a given password. Formulas of this type of algorithms can be variously constructed so that different search results can be obtained according to the same password. Added to this is the issue of promoting search results for companies that have made certain fees for a high level of positioning in search results. Unfortunately, this is not an objective process of finding information available on the Internet but a formula based on commercial marketing. In this situation, there is a question about competitiveness, which is limited in this way.