A study carried out by Jean Véronis and his students at the University of Aix into the relevance of search engine results shows there is very little difference between Yahoo! and Google .

Google, Yahoo and MSN, Exalead, Voilà and Dir.com were selected for the test because they all had French language versions. To make it a blind test the students were only fed the urls from each search so they did not know which engine the results came from, they then had to grade each document served with a mark from 0-5 based on relevance.

See the ranking results below:

Jean said "...the most striking result is undoubtedly the extremely low level of user satisfaction"

Google and Yahoo! also matched on the measure of Proximity, the number of results they share whilst MSN comes in a close third. This is a very good study and much like the one our MD Lucy Cokes did for her thesis back in 1996.  Download the PDF "A comparative study of six search engines" here.