| Year | Value | | 2019 | 50.648564124244 | | 2018 | 50.677813201321 | | 2017 | 50.71503929579 | | 2016 | 50.752138338456 | | 2015 | 50.782693358475 | | 2014 | 50.804548669648 | | 2013 | 50.818897967999 | | 2012 | 50.826419766442 | | 2011 | 50.829083635101 | | 2010 | 50.828528856834 | | 2009 | 50.82433736654 | | 2008 | 50.815950476275 | | 2007 | 50.80499148528 | | 2006 | 50.793673394587 | | 2005 | 50.783651513728 | | 2004 | 50.77579284454 | | 2003 | 50.76986473565 | | 2002 | 50.76500540169 | | 2001 | 50.759696930983 | | 2000 | 50.752764902879 | | 1999 | 50.744061506766 | | 1998 | 50.733973518362 | | 1997 | 50.722466333263 | | 1996 | 50.709614168373 | | 1995 | 50.695526022069 | | 1994 | 50.679852128256 | | 1993 | 50.662626243941 | | 1992 | 50.64486420963 | | 1991 | 50.628035673209 | | 1990 | 50.613138392113 | | 1989 | 50.601144739804 | | 1988 | 50.591590264251 | | 1987 | 50.582222280495 | | 1986 | 50.569575643803 | | 1985 | 50.55111612489 | | 1984 | 50.525485658504 | | 1983 | 50.49357738021 | | 1982 | 50.458069782364 | | 1981 | 50.423254557713 | | 1980 | 50.392646238723 | | 1979 | 50.367384616515 | | 1978 | 50.347049379159 | | 1977 | 50.331782226894 | | 1976 | 50.321169221032 |
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