What are the key aspects of Panda update ?

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Google’s Panda Update is a search filter introduced in February 2011 meant to stop sites with poor quality content from working their way into Google’s top search results. Panda is updated from time-to-time. When this happens, sites previously hit may escape, if they’ve made the right changes. Panda may also catch sites that escaped before. A refresh also means “false positives” might get released.