| Year | Value | | 2021 | 1.896 | | 2020 | 1.908 | | 2019 | 1.932 | | 2018 | 1.964 | | 2017 | 2.091 | | 2016 | 2.123 | | 2015 | 2.117 | | 2014 | 2.17 | | 2013 | 2.198 | | 2012 | 2.282 | | 2011 | 2.471 | | 2010 | 2.474 | | 2009 | 2.629 | | 2008 | 2.687 | | 2007 | 2.757 | | 2006 | 2.839 | | 2005 | 2.933 | | 2004 | 3.038 | | 2003 | 3.151 | | 2002 | 3.271 | | 2001 | 3.399 | | 2000 | 3.539 | | 1999 | 3.697 | | 1998 | 3.876 | | 1997 | 4.074 | | 1996 | 4.287 | | 1995 | 4.505 | | 1994 | 4.72 | | 1993 | 4.921 | | 1992 | 5.102 | | 1991 | 5.259 | | 1990 | 5.392 | | 1989 | 5.504 | | 1988 | 5.603 | | 1987 | 5.695 | | 1986 | 5.784 | | 1985 | 5.875 | | 1984 | 5.97 | | 1983 | 6.068 | | 1982 | 6.17 | | 1981 | 6.273 | | 1980 | 6.375 | | 1979 | 6.473 | | 1978 | 6.564 | | 1977 | 6.645 | | 1976 | 6.714 | | 1975 | 6.771 | | 1974 | 6.818 | | 1973 | 6.855 | | 1972 | 6.887 | | 1971 | 6.913 | | 1970 | 6.935 | | 1969 | 6.954 | | 1968 | 6.97 | | 1967 | 6.982 | | 1966 | 6.99 | | 1965 | 6.993 | | 1964 | 6.989 | | 1963 | 6.976 | | 1962 | 6.953 | | 1961 | 6.922 | | 1960 | 6.885 |
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