| Year | Value | | 2021 | 1.717 | | 2020 | 1.737 | | 2019 | 1.765 | | 2018 | 1.787 | | 2017 | 1.818 | | 2016 | 1.843 | | 2015 | 1.863 | | 2014 | 1.883 | | 2013 | 1.905 | | 2012 | 1.93 | | 2011 | 1.958 | | 2010 | 1.992 | | 2009 | 2.033 | | 2008 | 2.081 | | 2007 | 2.137 | | 2006 | 2.198 | | 2005 | 2.263 | | 2004 | 2.33 | | 2003 | 2.395 | | 2002 | 2.458 | | 2001 | 2.516 | | 2000 | 2.572 | | 1999 | 2.627 | | 1998 | 2.683 | | 1997 | 2.742 | | 1996 | 2.802 | | 1995 | 2.862 | | 1994 | 2.918 | | 1993 | 2.967 | | 1992 | 3.01 | | 1991 | 3.047 | | 1990 | 3.082 | | 1989 | 3.117 | | 1988 | 3.158 | | 1987 | 3.208 | | 1986 | 3.269 | | 1985 | 3.343 | | 1984 | 3.43 | | 1983 | 3.527 | | 1982 | 3.632 | | 1981 | 3.743 | | 1980 | 3.856 | | 1979 | 3.967 | | 1978 | 4.074 | | 1977 | 4.179 | | 1976 | 4.285 | | 1975 | 4.398 | | 1974 | 4.527 | | 1973 | 4.68 | | 1972 | 4.857 | | 1971 | 5.057 | | 1970 | 5.276 | | 1969 | 5.506 | | 1968 | 5.737 | | 1967 | 5.957 | | 1966 | 6.159 | | 1965 | 6.333 | | 1964 | 6.475 | | 1963 | 6.584 | | 1962 | 6.663 | | 1961 | 6.712 | | 1960 | 6.735 |
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