Furthermore, some advertisers try to increase traffic to their pages by taking elaborate measures to fool automated search engines. For example, an advertiser might label sports, beer, and swimsuits as keywords in its subject tag, thinking these topics are likely to arouse Web surfers' interests, while the true content of the page is classical clocks, a much less alluring topic. In fact, there are even companies whose sole purpose and means of profit is the manipulation of search engines. There are also search engines whose owners sell companies the right to be listed at the top of the retrieved documents list for particular queries (Marchiori, 1997).
There is an additional Web information retrieval challenge related to precision. Although the amount of accessible information continues to grow, user ability to look at documents has not. Users rarely look beyond the first 10 or 20 documents retrieved, and this user impatience means that search engine precision must increase just as rapidly as the number of documents is increasing.
Broken links also pose a special problem. Often the most appealing retrieved document turns out to be a broken link, which quickly causes user frustration. Most search engines use a "web crawling" technique to gather and store information about individual Web pages and documents, thus, in effect, removing broken links from their database of retrievable documents.