| Year | Value | | 2021 | 1.803 | | 2020 | 1.818 | | 2019 | 1.834 | | 2018 | 1.881 | | 2017 | 1.929 | | 2016 | 1.978 | | 2015 | 2.039 | | 2014 | 2.081 | | 2013 | 2.089 | | 2012 | 2.116 | | 2011 | 2.149 | | 2010 | 2.144 | | 2009 | 2.217 | | 2008 | 2.244 | | 2007 | 2.227 | | 2006 | 2.268 | | 2005 | 2.328 | | 2004 | 2.415 | | 2003 | 2.526 | | 2002 | 2.602 | | 2001 | 2.757 | | 2000 | 2.914 | | 1999 | 3.037 | | 1998 | 3.119 | | 1997 | 3.213 | | 1996 | 3.281 | | 1995 | 3.322 | | 1994 | 3.356 | | 1993 | 3.369 | | 1992 | 3.381 | | 1991 | 3.376 | | 1990 | 3.374 | | 1989 | 3.399 | | 1988 | 3.508 | | 1987 | 3.595 | | 1986 | 3.698 | | 1985 | 3.78 | | 1984 | 3.866 | | 1983 | 3.918 | | 1982 | 3.963 | | 1981 | 4.006 | | 1980 | 4.056 | | 1979 | 4.112 | | 1978 | 4.2 | | 1977 | 4.306 | | 1976 | 4.406 | | 1975 | 4.519 | | 1974 | 4.627 | | 1973 | 4.735 | | 1972 | 4.81 | | 1971 | 4.909 | | 1970 | 5.028 | | 1969 | 5.152 | | 1968 | 5.284 | | 1967 | 5.422 | | 1966 | 5.627 | | 1965 | 5.829 | | 1964 | 6.016 | | 1963 | 6.182 | | 1962 | 6.292 | | 1961 | 6.362 | | 1960 | 6.407 |
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