| Year | Value | | 2021 | 2.348 | | 2020 | 2.371 | | 2019 | 2.394 | | 2018 | 2.418 | | 2017 | 2.443 | | 2016 | 2.469 | | 2015 | 2.494 | | 2014 | 2.519 | | 2013 | 2.543 | | 2012 | 2.565 | | 2011 | 2.587 | | 2010 | 2.61 | | 2009 | 2.633 | | 2008 | 2.659 | | 2007 | 2.687 | | 2006 | 2.717 | | 2005 | 2.749 | | 2004 | 2.781 | | 2003 | 2.811 | | 2002 | 2.84 | | 2001 | 2.868 | | 2000 | 2.895 | | 1999 | 2.923 | | 1998 | 2.955 | | 1997 | 2.99 | | 1996 | 3.03 | | 1995 | 3.072 | | 1994 | 3.115 | | 1993 | 3.156 | | 1992 | 3.195 | | 1991 | 3.231 | | 1990 | 3.267 | | 1989 | 3.304 | | 1988 | 3.345 | | 1987 | 3.391 | | 1986 | 3.445 | | 1985 | 3.505 | | 1984 | 3.57 | | 1983 | 3.64 | | 1982 | 3.715 | | 1981 | 3.798 | | 1980 | 3.895 | | 1979 | 4.015 | | 1978 | 4.161 | | 1977 | 4.332 | | 1976 | 4.526 | | 1975 | 4.734 | | 1974 | 4.945 | | 1973 | 5.148 | | 1972 | 5.336 | | 1971 | 5.504 | | 1970 | 5.653 | | 1969 | 5.79 | | 1968 | 5.922 | | 1967 | 6.053 | | 1966 | 6.182 | | 1965 | 6.304 | | 1964 | 6.412 | | 1963 | 6.497 | | 1962 | 6.558 | | 1961 | 6.594 | | 1960 | 6.608 |
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