| Year | Value | | 2021 | 4.1230471895635 | | 2020 | 4.1794169753584 | | 2019 | 3.9821227250765 | | 2018 | 3.8975680463843 | | 2017 | 4.1592849090031 | | 2016 | 3.9297793525528 | | 2015 | 4.3002093734901 | | 2014 | 3.9056208729264 | | 2013 | 4.8075374456434 | | 2012 | 4.6219197938476 | | 2011 | 4.6533258173619 | | 2010 | 4.7769367047834 | | 2009 | 4.8292800773071 | | 2008 | 4.9766468030279 | | 2007 | 4.8115638589145 | | 2006 | 4.9645675632147 | | 2005 | 5.2182315992914 | | 2004 | 5.1417297471412 | | 2003 | 5.2303108391045 | | 2002 | 5.4759220486391 | | 2001 | 6.0396199065872 | | 2000 | 6.50668384603 | | 1999 | 6.4825253664036 | | 1998 | 6.4503140602351 | | 1997 | 5.812021384193 | | 1996 | 5.8047969946539 | | 1995 | 5.6855945672591 | | 1994 | 5.8553677214275 | | 1993 | 5.6675335934114 | | 1992 | 5.898714058662 | | 1991 | 5.938448201127 | | 1990 | 5.7939604103453 | | 1989 | 5.7181043201849 | | 1988 | 5.8698165005057 | | 1987 | 5.8373067475798 | | 1986 | 5.7975726051149 | | 1985 | 5.7433896835717 | | 1984 | 5.6892067620286 | | 1983 | 5.6350238404855 | | 1982 | 5.6061262823291 | | 1981 | 5.7795116312672 | | 1980 | 5.5700043346337 | | 1979 | 5.7072677358763 | | 1978 | 5.8156335789626 | | 1977 | 5.9601213697443 | | 1976 | 6.0684872128305 | | 1975 | 6.2129750036122 | | 1974 | 6.3321774310071 | | 1973 | 6.285218899003 | | 1972 | 6.2671579251553 | | 1971 | 6.2490969513076 | | 1970 | 6.2310359774599 | | 1969 | 6.2129750036122 | | 1968 | 6.1949140297645 | | 1967 | 6.2129750036122 | | 1966 | 6.1949140297645 | | 1965 | 6.1949140297645 | | 1964 | 6.1768530559168 | | 1963 | 6.1768530559168 | | 1962 | 6.1587920820691 | | 1961 | 6.1587920820691 |
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