| Year | Value | | 2020 | 6.306 | | 2019 | 6.247 | | 2018 | 6.161 | | 2017 | 6.102 | | 2016 | 6.137 | | 2015 | 5.91 | | 2014 | 5.883 | | 2013 | 5.84 | | 2012 | 5.823 | | 2011 | 5.829 | | 2010 | 5.803 | | 2009 | 5.819 | | 2008 | 5.711 | | 2007 | 5.609 | | 2006 | 5.366 | | 2005 | 4.999 | | 1999 | 4.195 | | 1998 | 4.091 | | 1997 | 3.957 | | 1996 | 3.842 | | 1995 | 3.841 | | 1994 | 3.803 | | 1993 | 3.798 | | 1992 | 3.697 | | 1991 | 3.584 | | 1990 | 3.333 | | 1987 | 3.3 | | 1986 | 3.1 | | 1985 | 2.9 | | 1984 | 2.9 | | 1983 | 2.8 | | 1982 | 2.6 | | 1981 | 2.5 | | 1980 | 2.4 | | 1979 | 2.3 | | 1978 | 2.3 | | 1977 | 2.2 | | 1976 | 2.1 | | 1975 | 2 | | 1974 | 2 | | 1973 | 1.9 | | 1972 | 1.7 | | 1971 | 1.7 | | 1970 | 1.6 | | 1969 | 1.6 | | 1968 | 1.5 | | 1967 | 1.5 | | 1966 | 1.4 | | 1965 | 1.4 | | 1964 | 1.4 | | 1963 | 1.3 | | 1962 | 1.3 | | 1961 | 1.2 | | 1960 | 1.3 |
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