By 2004, search engines had incorporated a wide range of undisclosed factors in their ranking algorithms to reduce the impact of link manipulation. In June 2007, The New York Times' Saul Hansell stated Google ranks sites using more than 200 different signals. The leading search engines, Google, Bing, and Yahoo, do not disclose the algorithms they use to rank pages. Some SEO practitioners have studied different approaches to search engine optimization, and have shared their personal opinions. Patents related to search engines can provide information to better understand search engines. In 2005, Google began personalizing search results for each user. Depending on their history of previous searches, Google crafted results for logged in users.
SEO often involves the concerted effort of multiple departments within an organization, including the design, marketing, and content production teams. While some SEO work entails business analysis (e.g., comparing one’s content with competitors’), a sizeable part depends on the ranking algorithms of various search engines, which may change with time. Nevertheless, a rule of thumb is that websites and webpages with higher-quality content, more external referral links, and more user engagement will rank higher on an SERP.