| Year | Value | | 2021 | 2.748 | | 2020 | 2.777 | | 2019 | 2.805 | | 2018 | 2.831 | | 2017 | 2.865 | | 2016 | 2.892 | | 2015 | 2.951 | | 2014 | 3.031 | | 2013 | 3.096 | | 2012 | 3.127 | | 2011 | 3.122 | | 2010 | 3.264 | | 2009 | 3.363 | | 2008 | 3.404 | | 2007 | 3.444 | | 2006 | 3.486 | | 2005 | 3.49 | | 2004 | 3.543 | | 2003 | 3.604 | | 2002 | 3.656 | | 2001 | 3.718 | | 2000 | 3.714 | | 1999 | 3.788 | | 1998 | 3.848 | | 1997 | 3.899 | | 1996 | 3.967 | | 1995 | 4.042 | | 1994 | 4.072 | | 1993 | 4.148 | | 1992 | 4.242 | | 1991 | 4.319 | | 1990 | 4.352 | | 1989 | 4.376 | | 1988 | 4.486 | | 1987 | 4.521 | | 1986 | 4.546 | | 1985 | 4.652 | | 1984 | 4.773 | | 1983 | 4.883 | | 1982 | 4.921 | | 1981 | 4.973 | | 1980 | 5.108 | | 1979 | 5.255 | | 1978 | 5.393 | | 1977 | 5.442 | | 1976 | 5.505 | | 1975 | 5.596 | | 1974 | 5.693 | | 1973 | 5.814 | | 1972 | 5.957 | | 1971 | 6.088 | | 1970 | 6.198 | | 1969 | 6.303 | | 1968 | 6.409 | | 1967 | 6.539 | | 1966 | 6.667 | | 1965 | 6.781 | | 1964 | 6.867 | | 1963 | 6.947 | | 1962 | 7.02 | | 1961 | 7.087 | | 1960 | 7.148 |
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