The most popular Web search engines rarely deliver the same results when identical queries are performed on each site, and the disparity has increased over the past two years, new research has found.

In a study of 19,332 queries, Google, Yahoo, Windows Live and Ask delivered the same top result only 3.6 percent of the time. The four engines never delivered the same top three results, even when the order of the results was ignored. Fewer than 1 percent of first-page results were shared by all four sites.

The search engines agreed more often in a study two years ago, in which the top result matched all four engines on 7 percent of queries.

"These differences contradict any notion that all search engines are the same and that searching one engine will yield the absolute best results of the Web," the paper states.

Dogpile.com, a metasearch engine that combines results from the top search sites, conducted the study with researchers from Queensland University of Technology and Pennsylvania State University. The research paper claims the study highlights the value of sites that deliver results from multiple search engines.

The research spurred an executive with Ask.com to discuss the importance of knowing the strengths and weaknesses of each search tool, even for users who search only one engine each time they need information.

"We like to think of search engine overlap with a 'classical' library mindset. Libraries often acquire (for a fee) more than one reference book or electronic database that contain the same data, journal indexing. However, each database or book might offer different features, different ways of indexing. In other words, it's worth the money to offer, know about, and use a variety of tools. No one resource is ideal for each query," states a posting this week on ResourceShelf, a daily electronic newsletter for online researchers. The newsletter is written by Gary Price, a librarian and director of online information for Ask.com.

Searchers can find a comparison of engines in PC World, which recently tested the Web's major search engines and pronounced Google the best of the lot. AlltheWeb and AltaVista, both owned by Yahoo, topped Google in text-search tests, but Google is able to deliver accurate and comprehensive results for text, video, images, news and blogs, the magazine said.

Most of the results Google and other engines deliver on the first page won't be found in the top results of the other sites. Nearly 70 percent of first-page results delivered by Google in the search overlap study were not found in the first-page results of Yahoo, Ask or Windows Live. About 80 percent of first-page results on Yahoo and Live were unique to those engines, compared to 75 percent of Ask's first-page results.

The study's 19,332 queries returned 776,435 unduplicated first-page results from the four engines. Out of those results, 0.6 percent were shared by all four search engines, and 88.3 percent were found only in one of the four.