A class of techniques, known as "Black Hat SEO" or spamdexing, use methods such as link farms, keyword stuffing, re-directs and duplicate/hidden content to trick search engine spiders into returning data that temporarily increase the page weight of a given web page in order to increase it's search result position for a given keyword or phrase. Search Engines depend on accurate search results for their users. These techniques have a negative effect on the accuracy of search results so search engines keep a sharp eye out them and remove the site in question when they are discovered.