| Year | Value | | 2021 | 2.01 | | 2020 | 2.03 | | 2019 | 2.05 | | 2018 | 2.06 | | 2017 | 2.08 | | 2016 | 2.09 | | 2015 | 2.11 | | 2014 | 2.09 | | 2013 | 2.08 | | 2012 | 2.06 | | 2011 | 2.16 | | 2010 | 2.3 | | 2009 | 2.37 | | 2008 | 2.44 | | 2007 | 2.51 | | 2006 | 2.03 | | 2005 | 2.24 | | 2004 | 2.17 | | 2003 | 2.07 | | 2002 | 2.21 | | 2001 | 1.9 | | 2000 | 2.06 | | 1999 | 2.377 | | 1998 | 2.501 | | 1997 | 2.727 | | 1996 | 2.559 | | 1995 | 2.717 | | 1994 | 3.044 | | 1993 | 3.096 | | 1992 | 3.204 | | 1991 | 3.102 | | 1990 | 2.99 | | 1989 | 2.95 | | 1988 | 2.804 | | 1987 | 2.845 | | 1986 | 2.74 | | 1985 | 2.821 | | 1984 | 2.847 | | 1983 | 2.996 | | 1982 | 2.952 | | 1981 | 2.89 | | 1980 | 3.094 | | 1979 | 3.025 | | 1978 | 3.304 | | 1977 | 3.268 | | 1976 | 3.424 | | 1975 | 3.627 | | 1974 | 3.863 | | 1973 | 4.142 | | 1972 | 4.639 | | 1971 | 4.898 | | 1970 | 5.127 | | 1969 | 4.667 | | 1968 | 4.744 | | 1967 | 4.866 | | 1966 | 4.568 | | 1965 | 5.113 | | 1964 | 5.016 | | 1963 | 4.648 | | 1962 | 4.929 | | 1961 | 4.893 | | 1960 | 5.445 |
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