| Year | Value | | 2023 | 3.638 | | 2022 | 3.65 | | 2021 | 5.349 | | 2020 | 8.055 | | 2019 | 3.669 | | 2018 | 3.896 | | 2017 | 4.355 | | 2016 | 4.869 | | 2015 | 5.28 | | 2014 | 6.168 | | 2013 | 7.375 | | 2012 | 8.069 | | 2011 | 8.949 | | 2010 | 9.633 | | 2009 | 9.254 | | 2008 | 5.784 | | 2007 | 4.622 | | 2006 | 4.623 | | 2005 | 5.084 | | 2004 | 5.529 | | 2003 | 5.989 | | 2002 | 5.783 | | 2001 | 4.731 | | 2000 | 3.992 | | 1999 | 4.219 | | 1998 | 4.511 | | 1997 | 5 | | 1996 | 5.451 | | 1995 | 5.65 | | 1994 | 6.119 | | 1993 | 6.9 | | 1992 | 7.5 | | 1991 | 6.8 | | 1990 | 5.6 | | 1989 | 5.3 | | 1988 | 5.5 | | 1987 | 6.2 | | 1986 | 7 | | 1985 | 7.2 | | 1984 | 7.5 | | 1983 | 9.6 | | 1982 | 9.7 | | 1981 | 7.6 | | 1980 | 7.1 | | 1979 | 5.8 | | 1978 | 6.1 | | 1977 | 7.1 | | 1976 | 7.7 | | 1975 | 8.5 | | 1974 | 5.6 | | 1973 | 4.9 | | 1972 | 5.6 | | 1971 | 5.9 | | 1970 | 4.9 | | 1969 | 3.5 | | 1968 | 3.6 | | 1967 | 3.8 | | 1966 | 3.8 | | 1965 | 4.5 | | 1964 | 5.2 | | 1963 | 5.7 | | 1962 | 5.5 | | 1961 | 6.7 | | 1960 | 5.5 |
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