Google has announced a new Google Panda algorithm with a version number of 24.
According to Google, the algorithm upgrade to 1.2% of the English site has a significant impact.
The last confirmed update was on December 21, 2012, with Google Panda24, which affected 1.3% of English-language websites. The earlier one occurred on November 21, affecting 0.8% of the search results. It looks like Google updated the algorithm every 4 weeks or so.
Here are all versions of the Google Panda algorithm so far:
Google Panda 1 February 24, 2011 update (affect 11.8% search results; announcement, only for U.S. English)
Google Panda 2 April 11, 2011 Update (Affect 2% of search results; announcement, launch International English)
Google Panda 3 May 10, 2011 update (unchanged; confirmed, not announced)
Google Panda 4 June 16, 2011 update (unchanged; confirmed, not announced)
Google Panda 5 July 23, 2011 update (unchanged; confirmed, not announced)
Google Panda 6 August 12, 2011 update (many non-English language search results that affect 6%-9%; published)
Google Panda 7 September 28, 2011 update (unchanged; confirmed, not announced)
Google Panda 8 October 19, 2011 update (affects about 2% of search results, later confirmed)
Google Panda 9 November 18, 2011 update (Impact less than 1% of search results; announcement)
Google Panda 10 January 18, 2012 update (no change; confirmed, not announced)
Google Panda 11 February 27, 2012 update (no change; announcement)
Google Panda 12 March 23, 2012 update (Impact of about 1.6% of search results; announcement)
Google Panda 13 April 19, 2012 update (no change, later confirmed)
Google Panda 14 April 27, 2012 update (no change; confirmation)
Google Panda 15 June 9, 2012 update (affects 1% of search results, later confirmed)
Google Panda 16 June 25, 2012 update (affects about 1% of search results; announcement)
Google Panda 17 July 24, 2012 update (affects about 1% of search results; announcement)
Google Panda 18 August 20, 2012 update (affects about 1% of search results; later confirmed)
Google Panda 19 September 18, 2012 update (impact less than 0.7% of search results; announcement)
Google Panda 20 September 27, 2012 update (affect 2.4% of English search results, later confirmed)
Google Panda 21 November 5, 2012 update (affecting 1.1% of U.S. English search results, affecting 0.4% of global search results; confirmed, not published)
Google Panda 22 November 21, 2012 update (affect 0.8% English search results; confirmed, not published)
Google Panda 23 December 21, 2012 update (affect the English search results of 1.3%; confirmation, announcement)
Google Panda 24 January 22, 2013 Update (affect 1.2% English search results; confirm, announce)
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