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I'm doing some research using various search engines - I use Google, Lycos, MSN, and Excite - I have a pretty good understanding of the feeders for each of these (Google:Google/Dmoz, MSN: Looksmart/Overture/Inktomi, etc.)
here's my question - it is my understanding that Google, Lycos, and MSN use the default operator of "AND" while Excite uses "OR", if I just type in two keywords without any operator in between. But, the first three return many many times the # of sites, vs. excite, which always returns a small #. This makes me think that either I don't understand "AND" vs. "OR", or that I have bad info about the search sites. Can anyone help?
However you have made some basic assumptions about the various search engines that can undermine your conclusion, basically:
1) Database size. If the database size of the various search engines is different, that is they have differing numbers of pages indexed, you will get different numbers of results, even when casting a wider OR net.
2) Algorithm cutoffs. Excite might have a relevancy cutoff point, higher than other search engines. If the search term does not appear high enough or repeated often enough, Excite might not include that page in the results page.