In a new study on factors that affect search engine rankings of Web pages, marketing analytics company Covario reported that the number of inbound links from other Web sites is the single most important factor in a page showing up prominently in search results.

Covario’s study—“A Statistical Analysis of Features Affecting Rank in Search Engine Results”—cited external inbound links from educational (.edu) sites, the Delicious social bookmarking site, government (.gov) sites, the blog indexer Technorati.com and Wikipedia, in that order, as returning particularly high page rankings.

In considering differences among the search engines, Covario reported that Google and Yahoo put more weight on external inbound links than did Microsoft’s Live Search. Yahoo and Microsoft, meanwhile, appeared to severely penalize pages with long URL names, while Google did not; although Google gave more weight to having large pages.

Covario’s study analyzed rank results using critical terms identified by its advertiser clients. More than 25,000 different URLs were analyzed.