| Year | Value | | 2021 | 1.52 | | 2020 | 1.7 | | 2019 | 1.81 | | 2018 | 1.73 | | 2017 | 1.75 | | 2016 | 1.9 | | 2015 | 1.94 | | 2014 | 1.97 | | 2013 | 1.98 | | 2012 | 2 | | 2011 | 1.96 | | 2010 | 1.92 | | 2009 | 1.82 | | 2008 | 1.9 | | 2007 | 1.97 | | 2006 | 1.97 | | 2005 | 2 | | 2004 | 2.0535 | | 2003 | 1.9 | | 2002 | 1.8 | | 2001 | 1.8 | | 2000 | 2 | | 1999 | 2.07 | | 1998 | 2 | | 1997 | 2.07 | | 1996 | 2.06 | | 1995 | 2.29 | | 1994 | 2.52 | | 1993 | 2.7 | | 1992 | 2.74 | | 1991 | 2.87 | | 1990 | 2.74 | | 1989 | 2.8 | | 1988 | 2.8 | | 1987 | 2.9026 | | 1986 | 2.9 | | 1985 | 2.91 | | 1984 | 3.259 | | 1983 | 3.293 | | 1982 | 3.343 | | 1981 | 3.41 | | 1980 | 3.497 | | 1979 | 3.605 | | 1978 | 3.73 | | 1977 | 3.869 | | 1976 | 4.018 | | 1975 | 4.176 | | 1974 | 4.33 | | 1973 | 4.451 | | 1972 | 4.615 | | 1971 | 4.769 | | 1970 | 4.913 | | 1969 | 5.065 | | 1968 | 5.222 | | 1967 | 5.377 | | 1966 | 5.663 | | 1965 | 5.792 | | 1964 | 5.888 | | 1963 | 5.945 | | 1962 | 5.961 | | 1961 | 5.937 | | 1960 | 5.878 |
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