| Year | Value | | 2022 | 2.5599883912743 | | 2021 | 2.6503759152215 | | 2020 | 2.6887876673069 | | 2019 | 2.7038755672529 | | 2018 | 2.7686811625809 | | 2017 | 2.8656554208403 | | 2016 | 2.9348112891248 | | 2015 | 2.9723456082163 | | 2014 | 2.9797784992387 | | 2013 | 3.0084219546281 | | 2012 | 3.0318800772248 | | 2011 | 2.969738308726 | | 2010 | 2.9392675583951 | | 2009 | 2.9314075345183 | | 2008 | 2.9435686352693 | | 2007 | 3.0327864114017 | | 2006 | 3.1224815364946 | | 2005 | 3.1489928059639 | | 2004 | 3.1380206522568 | | 2003 | 3.1045175126828 | | 2002 | 3.0747900351064 | | 2001 | 3.0407289464883 | | 2000 | 2.9838855873639 | | 1999 | 2.9251841648916 | | 1998 | 2.7488768804585 | | 1997 | 2.5666970413562 | | 1996 | 2.5551209567379 | | 1995 | 2.5632429424654 | | 1994 | 2.5205973315085 | | 1993 | 2.4850766675275 | | 1992 | 2.4626396482139 | | 1991 | 2.5271417328437 | | 1990 | 2.6199124814709 | | 1989 | 2.6371436145507 | | 1988 | 2.675494975703 | | 1987 | 2.7301030574997 | | 1986 | 2.8251935287994 | | 1985 | 2.8952999075693 | | 1984 | 2.8831009677501 | | 1983 | 2.8193745952606 | | 1982 | 2.7427056561404 | | 1981 | 2.714360454824 | | 1980 | 2.6754392298235 | | 1979 | 2.5836317442033 | | 1978 | 2.4813599193909 | | 1977 | 2.3144677291987 | | 1976 | 2.1167584031351 | | 1975 | 1.9792882203038 | | 1974 | 1.8514713529203 | | 1973 | 1.7495329076829 | | 1972 | 1.6935930972595 | | 1971 | 1.698478892082 | | 1970 | 1.7240552835628 | | 1969 | 1.7120393852742 | | 1968 | 1.7020457510888 | | 1967 | 1.6611106395776 | | 1966 | 1.6319992210558 | | 1965 | 1.5989725113869 | | 1964 | 1.5373340122322 | | 1963 | 1.4949939674489 | | 1962 | 1.4659479301649 | | 1961 | 1.4440544779797 |
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