| Year | Value | | 2022 | 3.4549197977344 | | 2021 | 3.4579772192345 | | 2020 | 3.6960098282307 | | 2019 | 3.4345677203884 | | 2018 | 3.3238664042118 | | 2017 | 3.3205409117412 | | 2016 | 3.4225879748699 | | 2015 | 3.4814282152278 | | 2014 | 3.6909607610934 | | 2013 | 4.0326610701753 | | 2012 | 4.4617099699335 | | 2011 | 4.8224418832637 | | 2010 | 4.9040233318294 | | 2009 | 4.8757683587977 | | 2008 | 4.4465955215626 | | 2007 | 4.0733505146769 | | 2006 | 4.0413423016604 | | 2005 | 4.0892318752451 | | 2004 | 4.0163127364069 | | 2003 | 3.8271610447362 | | 2002 | 3.4476180992878 | | 2001 | 3.123809802667 | | 2000 | 3.1122421469768 | | 1999 | 3.0856767832475 | | 1998 | 3.20155849897 | | 1997 | 3.4055622441886 | | 1996 | 3.5549822063522 | | 1995 | 3.8602457917389 | | 1994 | 4.2152646751779 | | 1993 | 4.6043502950764 | | 1992 | 4.9704668076828 | | 1991 | 4.8834293977631 | | 1990 | 5.6051752943879 | | 1989 | 5.8712060080465 | | 1988 | 6.0712777020948 | | 1987 | 6.4202740228849 | | 1986 | 6.6265226578984 | | 1985 | 6.4532192046686 | | 1984 | 6.236416529639 | | 1983 | 6.3211442597672 | | 1982 | 6.8140570944128 | | 1981 | 5.6465412562426 | | 1980 | 5.1535374665164 | | 1979 | 4.9519915352726 | | 1978 | 4.9430872479169 | | 1977 | 5.1556173512712 | | 1976 | 5.1910714290256 | | 1975 | 5.6226790961334 | | 1974 | 5.9541111967601 | | 1973 | 5.8938705911674 | | 1972 | 6.5197569243472 | | 1971 | 6.9430696093606 | | 1970 | 8.0327435840408 | | 1969 | 8.633263795091 | | 1968 | 9.2684542746879 | | 1967 | 9.4177959325514 | | 1966 | 8.4353002859451 | | 1965 | 7.587247177226 | | 1964 | 8.0512811057284 | | 1963 | 8.8318911861802 | | 1962 | 9.3316729454584 | | 1961 | 9.1560315001712 | | 1960 | 8.9931245871528 |
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