The formula Google uses to rank the resulting Web pages from a user's query. Along with the PageRank system that was Google's major contribution to evaluating website importance due to popularity, Google uses numerous other criteria that change periodically to deliver more relevant results as well as to prevent people from gaming the system. For example, if one criterion were the quantity of identical words in the page, search engine optimization (SEO) specialists might embed multiple occurrences of the word in the page that are invisible to the user but not to the search engine spider that crawls the Web. When Google finds that people are doing excessive "keyword stuffing," it may lessen the value of that criterion in the overall mix.Continual ChangesOver the years, Google has had numerous algorithm changes, many of which have names and multiple versions, including Boston, Caffeine, Cassandra, Dominic, Esmerelda, Fritz, Florida, Austin, Brandy, Allegra, Bourbon, Gilligan, Jagger, Big Daddy, Buffy, Dewey, Panda and Penguin. In 2013, the Hummingbird algorithm was a major change in order to deliver more targeted results for users.Some Gain, Some LoseWith each algorithm change, some percentage of websites gain in popularity, while others lose. Occasionally, even major sites take a huge hit, drawing 10 million page views one day and 5 million the day after an algorithm change. See page ranking, Google AMP, Web spam, spamdexing and Google.