| Year | Value | | 2020 | 0.32491817598687 | | 2019 | 0.3309906910575 | | 2018 | 0.33941008275031 | | 2017 | 0.33566343080875 | | 2016 | 0.32023408342937 | | 2015 | 0.32807933955792 | | 2014 | 0.2721314834507 | | 2013 | 0.27872041384958 | | 2012 | 0.28481715687919 | | 2011 | 0.29110886673762 | | 2010 | 0.29914984384286 | | 2009 | 0.30712069570345 | | 2008 | 0.31513688846131 | | 2007 | 0.32330172646515 | | 2006 | 0.33167176697315 | | 2005 | 0.3381743161732 | | 2004 | 0.34460202041888 | | 2003 | 0.35452113626299 | | 2002 | 0.36362965744455 | | 2001 | 0.37316719976101 | | 2000 | 0.2542502828803 | | 1999 | 0.30376803919764 | | 1998 | 0.26910914485942 | | 1997 | 0.27641444176076 | | 1996 | 0.23648859190711 | | 1995 | 0.2430605463821 | | 1994 | 0.22422832452756 | | 1993 | 0.2190118921152 | | 1992 | 0.22516886628009 | | 1991 | 0.28619531131093 |
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