| Year | Value | | 2021 | 22.006052222071 | | 2020 | 22.020921732853 | | 2019 | 22.201192006778 | | 2018 | 22.653346708247 | | 2017 | 22.370587976221 | | 2016 | 22.33199324761 | | 2015 | 22.788719105427 | | 2014 | 22.647399066455 | | 2013 | 22.662092085829 | | 2012 | 20.770162403979 | | 2011 | 21.133339122973 | | 2010 | 19.642809410676 | | 2009 | 19.057702101201 | | 2008 | 21.046826737995 | | 2007 | 21.347297115583 | | 2006 | 21.125162349927 | | 2005 | 20.581928391458 | | 2004 | 19.97952779638 | | 2003 | 19.881170572105 | | 2002 | 20.827502717334 | | 2001 | 20.514041726191 | | 2000 | 20.878403516053 | | 1999 | 20.934255955552 | | 1998 | 20.412659046248 | | 1997 | 20.264301243729 | | 1996 | 20.516758429649 | | 1995 | 19.694930021162 | | 1994 | 19.353738108611 | | 1993 | 18.586618072003 | | 1992 | 20.026263031397 | | 1991 | 18.291138236045 | | 1990 | 17.668502023029 | | 1989 | 17.73912160779 | | 1988 | 16.986732069381 | | 1987 | 15.574426854669 | | 1986 | 17.551254225476 | | 1985 | 17.615918285222 | | 1984 | 18.010634411231 | | 1983 | 18.337974021782 | | 1982 | 16.594717893281 | | 1981 | 16.091362718626 | | 1980 | 15.180851237502 | | 1979 | 13.767688348265 | | 1978 | 13.456973828758 | | 1977 | 14.112765323655 | | 1976 | 13.843788935684 | | 1975 | 12.314150173683 | | 1974 | 11.779689116903 | | 1973 | 11.486338479365 |
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