| Year | Value | | 2021 | 55.393019913595 | | 2020 | 55.393019913595 | | 2019 | 54.847386393601 | | 2018 | 53.694038850312 | | 2017 | 53.106987342885 | | 2016 | 54.311075854795 | | 2015 | 54.564096422607 | | 2014 | 54.384524918696 | | 2013 | 54.773121648941 | | 2012 | 54.277658433682 | | 2011 | 54.090910169641 | | 2010 | 54.497837742814 | | 2009 | 54.102422870704 | | 2008 | 52.75890261053 | | 2007 | 54.813448184935 | | 2006 | 52.930439922651 | | 2005 | 53.263512349477 | | 2004 | 52.16477937945 | | 2003 | 55.33589346928 | | 2002 | 57.354660718995 | | 2001 | 56.684674342973 | | 2000 | 56.102312120946 | | 1999 | 57.394525797662 | | 1998 | 57.067815329173 | | 1997 | 59.693658257889 | | 1996 | 58.451261316692 | | 1995 | 57.572294981102 | | 1994 | 56.693328645513 | | 1993 | 56.253845477718 | | 1992 | 55.902258943483 | | 1991 | 55.902258943483 | | 1990 | 55.497934429111 | | 1989 | 55.09360991474 | | 1988 | 55.128768568164 | | 1987 | 54.724444053793 | | 1986 | 54.320119539422 | | 1985 | 54.355278192845 | | 1984 | 53.950953678474 | | 1983 | 53.107145996308 | | 1982 | 53.142304649732 | | 1981 | 52.298496967566 | | 1980 | 52.298496967566 | | 1979 | 52.298496967566 | | 1978 | 52.298496967566 | | 1977 | 51.859013799772 | | 1976 | 51.859013799772 | | 1975 | 51.859013799772 | | 1974 | 51.419530631977 | | 1973 | 51.419530631977 | | 1972 | 51.419530631977 | | 1971 | 51.419530631977 | | 1970 | 51.419530631977 | | 1969 | 51.419530631977 | | 1968 | 51.419530631977 | | 1967 | 51.419530631977 | | 1966 | 51.419530631977 | | 1965 | 51.419530631977 | | 1964 | 51.419530631977 | | 1963 | 51.419530631977 | | 1962 | 51.419530631977 | | 1961 | 51.419530631977 |
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