| Year | Value | | 2021 | 7.4983187626093 | | 2020 | 7.4983187626093 | | 2019 | 7.4983187626093 | | 2018 | 7.5857431069267 | | 2017 | 7.6126429051782 | | 2016 | 7.6395427034297 | | 2015 | 7.6664425016812 | | 2014 | 7.6933422999328 | | 2013 | 7.7202420981843 | | 2012 | 7.7471418964358 | | 2011 | 7.7740416946873 | | 2010 | 7.8009414929388 | | 2009 | 7.8278412911903 | | 2008 | 7.8547410894418 | | 2007 | 7.8816408876933 | | 2006 | 7.9085406859449 | | 2005 | 7.9354404841964 | | 2004 | 7.9623402824479 | | 2003 | 7.9892400806994 | | 2002 | 8.0161398789509 | | 2001 | 8.0430396772024 | | 2000 | 8.0699394754539 | | 1999 | 8.0699394754539 | | 1998 | 8.4061869535978 | | 1997 | 8.5406859448554 | | 1996 | 8.7424344317418 | | 1995 | 8.7424344317418 | | 1994 | 8.7424344317418 | | 1993 | 8.4061869535978 | | 1992 | 8.0699394754539 | | 1991 | 8.0699394754539 | | 1990 | 7.3974445191661 | | 1989 | 6.7249495628783 | | 1988 | 6.7249495628783 | | 1987 | 6.7249495628783 | | 1986 | 6.7249495628783 | | 1985 | 6.7249495628783 | | 1984 | 6.7249495628783 | | 1983 | 6.7249495628783 | | 1982 | 6.7249495628783 | | 1981 | 6.7249495628783 | | 1980 | 6.7249495628783 | | 1979 | 6.7249495628783 | | 1978 | 6.4559515803632 | | 1977 | 6.0524546065904 | | 1976 | 5.9179556153329 | | 1975 | 5.7162071284465 | | 1974 | 5.7162071284465 | | 1973 | 5.7162071284465 | | 1972 | 5.3799596503026 | | 1971 | 5.2454606590451 | | 1970 | 5.2454606590451 | | 1969 | 4.9092131809011 | | 1968 | 4.8419636852724 | | 1967 | 4.8419636852724 | | 1966 | 4.7074646940148 | | 1965 | 4.7074646940148 | | 1964 | 4.7074646940148 | | 1963 | 4.7074646940148 | | 1962 | 4.7074646940148 | | 1961 | 4.7074646940148 |
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