| Year | Value | | 2021 | 6.678 | | 2020 | 6.46 | | 2019 | 6.006 | | 2018 | 6.06 | | 2017 | 6.032 | | 2016 | 5.964 | | 2015 | 5.883 | | 2014 | 5.827 | | 2013 | 5.803 | | 2012 | 5.802 | | 2011 | 5.84 | | 2010 | 5.821 | | 2009 | 5.831 | | 2008 | 5.831 | | 2007 | 5.859 | | 2006 | 5.873 | | 2005 | 5.933 | | 2004 | 6.089 | | 2003 | 6.091 | | 2002 | 6.202 | | 2001 | 6.201 | | 2000 | 6.169 | | 1999 | 6.092 | | 1998 | 6.025 | | 1997 | 5.878 | | 1996 | 5.784 | | 1995 | 5.695 | | 1994 | 5.624 | | 1993 | 5.694 | | 1992 | 5.79 | | 1991 | 5.92 | | 1990 | 6.078 | | 1989 | 6.258 | | 1988 | 6.452 | | 1987 | 6.678 | | 1986 | 6.941 | | 1985 | 7.227 | | 1984 | 7.524 | | 1983 | 7.808 | | 1982 | 8.084 | | 1981 | 8.389 | | 1980 | 8.712 | | 1979 | 9.167 | | 1978 | 9.117 | | 1977 | 9.356 | | 1976 | 9.616 | | 1975 | 9.898 | | 1974 | 10.208 | | 1973 | 10.518 | | 1972 | 10.808 | | 1971 | 11.105 | | 1970 | 11.492 | | 1969 | 11.985 | | 1968 | 12.513 | | 1967 | 13.066 | | 1966 | 13.619 | | 1965 | 15.138 | | 1964 | 14.657 | | 1963 | 15.321 | | 1962 | 15.766 | | 1961 | 16.33 | | 1960 | 16.894 |
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