| Year | Value | | 2021 | 4.089 | | 2020 | 4.174 | | 2019 | 4.257 | | 2018 | 4.336 | | 2017 | 4.396 | | 2016 | 4.461 | | 2015 | 4.523 | | 2014 | 4.622 | | 2013 | 4.749 | | 2012 | 4.881 | | 2011 | 4.97 | | 2010 | 5.058 | | 2009 | 5.174 | | 2008 | 5.328 | | 2007 | 5.462 | | 2006 | 5.53 | | 2005 | 5.519 | | 2004 | 5.506 | | 2003 | 5.547 | | 2002 | 5.653 | | 2001 | 5.774 | | 2000 | 5.875 | | 1999 | 5.953 | | 1998 | 6.017 | | 1997 | 6.079 | | 1996 | 6.129 | | 1995 | 6.173 | | 1994 | 6.21 | | 1993 | 6.246 | | 1992 | 6.277 | | 1991 | 6.323 | | 1990 | 6.37 | | 1989 | 6.434 | | 1988 | 6.507 | | 1987 | 6.572 | | 1986 | 6.638 | | 1985 | 6.693 | | 1984 | 6.728 | | 1983 | 6.76 | | 1982 | 6.788 | | 1981 | 6.834 | | 1980 | 6.872 | | 1979 | 6.919 | | 1978 | 6.884 | | 1977 | 6.841 | | 1976 | 6.793 | | 1975 | 6.744 | | 1974 | 6.691 | | 1973 | 6.651 | | 1972 | 6.599 | | 1971 | 6.542 | | 1970 | 6.578 | | 1969 | 6.644 | | 1968 | 6.657 | | 1967 | 6.65 | | 1966 | 6.628 | | 1965 | 6.617 | | 1964 | 6.585 | | 1963 | 6.556 | | 1962 | 6.504 | | 1961 | 6.446 | | 1960 | 6.391 |
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