| Year | Value | | 2021 | 8.124 | | 2020 | 7.575 | | 2019 | 7.017 | | 2018 | 6.864 | | 2017 | 6.735 | | 2016 | 6.574 | | 2015 | 5.996 | | 2014 | 5.971 | | 2013 | 5.711 | | 2012 | 5.683 | | 2011 | 5.649 | | 2010 | 5.568 | | 2009 | 5.62 | | 2008 | 5.6 | | 2007 | 5.242 | | 2006 | 5.289 | | 2005 | 5.189 | | 2004 | 5.217 | | 2003 | 5.384 | | 2002 | 5.01 | | 2001 | 5.557 | | 2000 | 4.964 | | 1999 | 4.982 | | 1998 | 4.758 | | 1997 | 4.677 | | 1996 | 4.835 | | 1995 | 4.766 | | 1994 | 4.82 | | 1993 | 4.745 | | 1992 | 4.741 | | 1991 | 4.688 | | 1990 | 4.753 | | 1989 | 4.779 | | 1988 | 4.777 | | 1987 | 4.811 | | 1986 | 4.776 | | 1985 | 4.879 | | 1984 | 4.897 | | 1983 | 4.948 | | 1982 | 5.079 | | 1981 | 5.154 | | 1980 | 5.362 | | 1979 | 5.406 | | 1978 | 5.481 | | 1977 | 5.58 | | 1976 | 5.704 | | 1975 | 5.854 | | 1974 | 6.036 | | 1973 | 6.244 | | 1972 | 6.441 | | 1971 | 6.642 | | 1970 | 6.912 | | 1969 | 7.355 | | 1968 | 7.528 | | 1967 | 7.814 | | 1966 | 8.088 | | 1965 | 8.38 | | 1964 | 8.712 | | 1963 | 8.985 | | 1962 | 9.338 | | 1961 | 9.501 | | 1960 | 9.938 |
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