| Year | Value | | 2020 | 7.062 | | 2019 | 4.29 | | 2018 | 4.327 | | 2017 | 4.27 | | 2016 | 4.221 | | 2015 | 4.155 | | 2014 | 4.089 | | 2013 | 4.024 | | 2012 | 3.954 | | 2011 | 3.882 | | 2010 | 3.807 | | 2009 | 3.75 | | 2008 | 3.679 | | 2007 | 3.62 | | 2006 | 3.534 | | 2005 | 3.447 | | 2004 | 3.37 | | 2003 | 3.283 | | 2002 | 3.213 | | 2001 | 3.122 | | 2000 | 3.027 | | 1999 | 2.97 | | 1998 | 2.928 | | 1997 | 2.867 | | 1996 | 2.856 | | 1995 | 2.839 | | 1994 | 3.68 | | 1993 | 3.58 | | 1992 | 3.479 | | 1991 | 3.37 | | 1990 | 3.253 | | 1989 | 2.8 | | 1988 | 2.8 | | 1987 | 2.8 | | 1986 | 2.8 | | 1985 | 2.6 | | 1984 | 2.6 | | 1983 | 2.4 | | 1982 | 2.3 | | 1981 | 2.3 | | 1980 | 2.2 | | 1979 | 2 | | 1978 | 2 | | 1977 | 1.9 | | 1976 | 1.8 | | 1975 | 1.7 | | 1974 | 1.6 | | 1973 | 1.5 | | 1972 | 1.5 | | 1971 | 1.4 | | 1970 | 1.3 | | 1969 | 1.3 | | 1968 | 1.2 | | 1967 | 1.2 | | 1966 | 1.1 | | 1965 | 1.1 | | 1964 | 1.1 | | 1963 | 1 | | 1962 | 1 | | 1961 | 1 | | 1960 | 1 |
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