| Year | Value | | 2021 | 2.804 | | 2020 | 2.848 | | 2019 | 2.891 | | 2018 | 2.929 | | 2017 | 2.972 | | 2016 | 3.011 | | 2015 | 3.066 | | 2014 | 3.112 | | 2013 | 3.178 | | 2012 | 3.237 | | 2011 | 3.305 | | 2010 | 3.39 | | 2009 | 3.487 | | 2008 | 3.589 | | 2007 | 3.724 | | 2006 | 3.855 | | 2005 | 3.992 | | 2004 | 4.113 | | 2003 | 4.247 | | 2002 | 4.379 | | 2001 | 4.496 | | 2000 | 4.578 | | 1999 | 4.67 | | 1998 | 4.777 | | 1997 | 4.957 | | 1996 | 5.153 | | 1995 | 5.34 | | 1994 | 5.529 | | 1993 | 5.663 | | 1992 | 5.774 | | 1991 | 5.877 | | 1990 | 5.982 | | 1989 | 6.055 | | 1988 | 6.117 | | 1987 | 6.171 | | 1986 | 6.219 | | 1985 | 6.263 | | 1984 | 6.328 | | 1983 | 6.387 | | 1982 | 6.443 | | 1981 | 6.491 | | 1980 | 6.553 | | 1979 | 6.607 | | 1978 | 6.654 | | 1977 | 6.694 | | 1976 | 6.73 | | 1975 | 6.769 | | 1974 | 6.8 | | 1973 | 6.823 | | 1972 | 6.853 | | 1971 | 6.858 | | 1970 | 6.857 | | 1969 | 6.847 | | 1968 | 6.852 | | 1967 | 6.846 | | 1966 | 6.854 | | 1965 | 6.845 | | 1964 | 6.846 | | 1963 | 6.837 | | 1962 | 6.836 | | 1961 | 6.829 | | 1960 | 6.828 |
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