| Year | Value | | 2021 | 6.719 | | 2020 | 7.534 | | 2019 | 4.913 | | 2018 | 4.911 | | 2017 | 4.872 | | 2016 | 4.86 | | 2015 | 4.773 | | 2014 | 4.752 | | 2013 | 4.762 | | 2012 | 4.793 | | 2011 | 4.826 | | 2010 | 4.852 | | 2009 | 4.856 | | 2008 | 4.847 | | 2007 | 4.817 | | 2006 | 4.794 | | 2005 | 4.78 | | 2004 | 4.788 | | 2003 | 4.821 | | 2002 | 4.875 | | 2001 | 4.935 | | 2000 | 5.007 | | 1999 | 5.074 | | 1998 | 5.157 | | 1997 | 5.222 | | 1996 | 5.275 | | 1995 | 5.33 | | 1994 | 5.375 | | 1993 | 5.468 | | 1992 | 5.498 | | 1991 | 5.627 | | 1990 | 5.678 | | 1989 | 5.851 | | 1988 | 6.073 | | 1987 | 6.345 | | 1986 | 6.655 | | 1985 | 6.979 | | 1984 | 7.291 | | 1983 | 7.614 | | 1982 | 7.839 | | 1981 | 8.078 | | 1980 | 8.328 | | 1979 | 8.595 | | 1978 | 8.892 | | 1977 | 9.23 | | 1976 | 9.621 | | 1975 | 10.029 | | 1974 | 10.475 | | 1973 | 10.927 | | 1972 | 11.358 | | 1971 | 11.765 | | 1970 | 12.138 | | 1969 | 12.522 | | 1968 | 12.726 | | 1967 | 12.994 | | 1966 | 13.267 | | 1965 | 13.553 | | 1964 | 13.869 | | 1963 | 14.217 | | 1962 | 14.596 | | 1961 | 14.977 | | 1960 | 15.371 |
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