| Year | Value | | 2021 | 1.595 | | 2020 | 1.597 | | 2019 | 1.611 | | 2018 | 1.617 | | 2017 | 1.629 | | 2016 | 1.657 | | 2015 | 1.694 | | 2014 | 1.734 | | 2013 | 1.772 | | 2012 | 1.824 | | 2011 | 1.871 | | 2010 | 1.915 | | 2009 | 1.96 | | 2008 | 2.027 | | 2007 | 2.094 | | 2006 | 2.158 | | 2005 | 2.105 | | 2004 | 2.125 | | 2003 | 2.169 | | 2002 | 2.276 | | 2001 | 2.542 | | 2000 | 2.351 | | 1999 | 2.554 | | 1998 | 2.333 | | 1997 | 2.392 | | 1996 | 2.519 | | 1995 | 2.673 | | 1994 | 2.874 | | 1993 | 3.102 | | 1992 | 3.281 | | 1991 | 3.067 | | 1990 | 2.96 | | 1989 | 2.948 | | 1988 | 3.016 | | 1987 | 2.797 | | 1986 | 2.959 | | 1985 | 2.934 | | 1984 | 2.906 | | 1983 | 3.165 | | 1982 | 2.961 | | 1981 | 2.961 | | 1980 | 2.995 | | 1979 | 3.034 | | 1978 | 3.108 | | 1977 | 3.215 | | 1976 | 3.359 | | 1975 | 3.576 | | 1974 | 3.844 | | 1973 | 4.135 | | 1972 | 4.475 | | 1971 | 4.808 | | 1970 | 5.135 | | 1969 | 5.46 | | 1968 | 4.964 | | 1967 | 5.138 | | 1966 | 5.858 | | 1965 | 4.841 | | 1964 | 4.894 | | 1963 | 5.272 | | 1962 | 5.21 | | 1961 | 5.278 | | 1960 | 6.866 |
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