In a recent post on the performance of MWI’s search engine optimization page a commenter brought up the point that MWI’s page has underscores in it vs dashes or hyphens, and that he had heard that dashes or hyphens are better for SEO than underscores.
An article by SEO expert Matt Cutts from August 2005 supports that viewpoint with technical information to back it up. I have not done any scientific experiments to test this out, but a quick test seems to indicate that Google, although it may have cared in the past, does not currently discriminate between underscores and dashes except in that it will not match a search that has underscores in it for a filename or path that has dashes and vice versa as I detail below.
As near as I can tell Google indexes a page with the part of the filename “search-engine-optimization” the same as “searchengineoptimization” or “search-engine-optimization” as evidenced by looking at the search results for the following:
We can see that the url is highlighted (bolded) in every case except for the search for “utah search-engine-optimization,” although even in that case the page ranks quite high. I believe of greatest interest is that even for the search “utah search engine” the part of the url “search_engine” is highlighted in the search results, implying that Google is ignoring underscores except in cases where the search uses another symbol in place of the underscore.
Therefore, the only justification I can see for using dashes instead of underscores is that in some rare cases someone might type in a search term with dashes, in which case a filename with dashes is likely to come up higher than a filename with underscores. Since the chances of someone typing in a search with underscores between words is probably zero, while the chance of someone typing a search with dashes might be one out of a thousand (although it could be higher in specific, non-normal cases), there is a slight justification there for using dashses.
This is just my own experience. If anyone has experiences that differ or hard evidence to support the use of dashes vs underscores I would like to hear them.