| Year | Value | | 2021 | 6.4 | | 2020 | 5.5 | | 2019 | 4.7 | | 2018 | 4 | | 2017 | 4.2 | | 2016 | 4.1 | | 2015 | 5.4 | | 2014 | 4.7 | | 2013 | 4.7 | | 2012 | 5.4 | | 2011 | 5.1 | | 2010 | 4.1 | | 2009 | 3.1 | | 2008 | 3.8 | | 2007 | 3.2 | | 2006 | 3.6 | | 2005 | 3.4 | | 2004 | 4.171 | | 2003 | 4.106 | | 2002 | 3.791 | | 2001 | 3.829 | | 2000 | 3.745 | | 1999 | 3.484 | | 1998 | 3.889 | | 1997 | 3.709 | | 1996 | 3.911 | | 1995 | 3.975 | | 1994 | 3.916 | | 1993 | 3.863 | | 1992 | 3.756 | | 1991 | 3.894 | | 1990 | 3.744 | | 1989 | 3.685 | | 1988 | 3.57 | | 1987 | 3.801 | | 1986 | 3.827 | | 1985 | 3.844 | | 1984 | 3.872 | | 1983 | 3.918 | | 1982 | 4.108 | | 1981 | 4.628 | | 1980 | 4.722 | | 1979 | 4.498 | | 1978 | 4.375 | | 1977 | 4.318 | | 1976 | 4.237 | | 1975 | 4.37 | | 1974 | 4.575 | | 1973 | 4.791 | | 1972 | 5.064 | | 1971 | 5.157 | | 1970 | 5.022 | | 1969 | 5.809 | | 1968 | 6.57 | | 1967 | 6.71 | | 1966 | 7.337 | | 1965 | 8.514 | | 1964 | 7.98 | | 1963 | 8.432 | | 1962 | 9.727 | | 1961 | 10.844 | | 1960 | 11.005 |
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