| Year | Value | | 2021 | 6.4845767861003 | | 2020 | 6.3707352221103 | | 2019 | 6.4343562442628 | | 2018 | 6.4568199081154 | | 2017 | 6.4544106968978 | | 2016 | 6.4747551471794 | | 2015 | 6.4675052060154 | | 2014 | 6.4594745019569 | | 2013 | 6.4515553354548 | | 2012 | 6.4436361689526 | | 2011 | 6.4356835411836 | | 2010 | 6.5004087851441 | | 2009 | 6.5646544176122 | | 2008 | 6.6289000500804 | | 2007 | 6.6931456825485 | | 2006 | 6.7573913150166 | | 2005 | 6.8769595754434 | | 2004 | 6.8931325211168 | | 2003 | 6.9093054667902 | | 2002 | 6.9254784124636 | | 2001 | 6.8381624410663 | | 2000 | 6.8358022171436 | | 1999 | 6.8331253157907 | | 1998 | 6.8304484144379 | | 1997 | 6.827771513085 | | 1996 | 6.8252061492886 | | 1995 | 6.832902240678 | | 1994 | 6.840486794511 | | 1993 | 6.8480713483441 | | 1992 | 6.8556559021771 | | 1991 | 6.863267938864 | | 1990 | 6.8487405736823 | | 1989 | 6.8346868415799 | | 1988 | 6.8206331094775 | | 1987 | 6.806579377375 | | 1986 | 6.7925135992166 | | 1985 | 6.8013371122257 | | 1984 | 6.8100370416225 | | 1983 | 6.8187369710192 | | 1982 | 6.8274369004159 | | 1981 | 6.8361368298127 | | 1980 | 6.8774057256689 | | 1979 | 6.919232309307 | | 1978 | 6.9610588929451 | | 1977 | 7.0028854765833 | | 1976 | 7.0445994072894 | | 1975 | 7.0414774710867 | | 1974 | 7.0384659570647 | | 1973 | 7.0354544430428 | | 1972 | 7.0324429290208 | | 1971 | 7.0294314149989 | | 1970 | 7.0448235977777 | | 1969 | 7.0598811678874 | | 1968 | 7.0749387379972 | | 1967 | 7.0899963081069 | | 1966 | 7.1050538782166 | | 1965 | 7.0927847470161 | | 1964 | 7.0440428348832 | | 1963 | 6.9953009227502 | | 1962 | 6.9465590106173 | | 1961 | 6.8978170984843 |
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