Search engines find information in many ways. In fact, there are about 40 different retrieval algorithms alone, most of which have been around for decades. We’re not going to cover them all here; if you’d like to learn more, read any of the standard texts on information retrieval.
We bring up the topic because it’s important to realize that a retrieval algorithm is essentially a tool, and just like other tools, specific algorithms help solve specific problems. And as retrieval algorithms are at the heart of search engines, it’s important to note that there is absolutely no single search engine that will meet all of your users’ information needs. Remember that fact the next time you hear a search engine vendor claim that his product’s brand-new proprietary algorithm is the solution to all information problems.
Most retrieval algorithms employ pattern matching; that is, they compare the user’s query with an index or, occasionally, with the full texts of documents, looking for the same string of text. When it finds a match, the source document is added to the retrieval set. So a user types the textual query “electric guitar,” and documents that include the text string “electric guitar” are retrieved. It all sounds quite simple. But this matching process can work in many different ways to produce different results.
Some algorithms return numerous results of varying relevance, while some return just a few high quality ...