| Year | Value | | 2021 | 4.7373828756058 | | 2020 | 4.7138287560582 | | 2019 | 4.7218093699515 | | 2018 | 4.7169305331179 | | 2017 | 4.6911793214863 | | 2016 | 4.6397415185783 | | 2015 | 4.6332794830372 | | 2014 | 4.5873990306947 | | 2013 | 4.5864297253635 | | 2012 | 4.6003231017771 | | 2011 | 4.6423263327948 | | 2010 | 4.6630048465267 | | 2009 | 4.6991922455573 | | 2008 | 4.708239095315 | | 2007 | 4.7334410339257 | | 2006 | 4.7269789983845 | | 2005 | 4.7334410339257 | | 2004 | 4.7560581583199 | | 2003 | 4.4361873990307 | | 2002 | 4.1143780290792 | | 2001 | 3.7919224555735 | | 2000 | 3.7899838449111 | | 1999 | 3.4604200323102 | | 1998 | 3.4604200323102 | | 1997 | 3.4571890145396 | | 1996 | 3.4604200323102 | | 1995 | 3.4604200323102 | | 1994 | 3.4636510500808 | | 1993 | 3.4636510500808 | | 1992 | 3.4991922455573 | | 1991 | 3.4894991922456 | | 1990 | 3.4894991922456 | | 1989 | 3.4798061389338 | | 1988 | 3.4668820678514 | | 1987 | 3.4571890145396 | | 1986 | 3.4474959612278 | | 1985 | 3.4281098546042 | | 1984 | 3.421647819063 | | 1983 | 3.4087237479806 | | 1982 | 3.3957996768982 | | 1981 | 3.3957996768982 | | 1980 | 3.3957996768982 | | 1979 | 3.3957996768982 | | 1978 | 3.3957996768982 | | 1977 | 3.389337641357 | | 1976 | 3.3861066235864 | | 1975 | 3.3828756058158 | | 1974 | 3.3796445880452 | | 1973 | 3.3764135702746 | | 1972 | 3.373182552504 | | 1971 | 3.3667205169628 | | 1970 | 3.3667205169628 | | 1969 | 3.3602584814217 | | 1968 | 3.3602584814217 | | 1967 | 3.3537964458804 | | 1966 | 3.3473344103393 | | 1965 | 3.3473344103393 | | 1964 | 3.3473344103393 | | 1963 | 3.3441033925687 | | 1962 | 3.3441033925687 | | 1961 | 3.3441033925687 |
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