Year | Value | 2022 | 17.341106166608 | 2021 | 18.693833248826 | 2020 | 17.374280536624 | 2019 | 16.238785206068 | 2018 | 16.151700592669 | 2017 | 16.708653537242 | 2016 | 16.601149029938 | 2015 | 16.76911556076 | 2014 | 16.73262540793 | 2013 | 17.942525931013 | 2012 | 18.864939184155 | 2011 | 19.679952043338 | 2010 | 19.506751063074 | 2009 | 18.772370177437 | 2008 | 21.046298208713 | 2007 | 21.724459741645 | 2006 | 21.470566853767 | 2005 | 19.577767500945 | 2004 | 19.793751394755 | 2003 | 18.742371472748 | 2002 | 18.43647320686 | 2001 | 16.853837639178 | 2000 | 17.489274749523 | 1999 | 17.1047699146 | 1998 | 17.591695816675 | 1997 | 17.774404276742 | 1996 | 17.862770184415 | 1995 | 17.515682821532 | 1994 | 18.590625500259 | 1993 | 18.521403200351 | 1992 | 17.963808448272 | 1991 | 18.239100895433 | 1990 | 19.152088757386 | 1989 | 24.992481140507 | 1988 | 22.20505893804 | 1987 | 21.366900916028 | 1986 | 17.95090946604 | 1985 | 20.879788673249 | 1984 | 18.855864972611 | 1983 | 17.182271757594 | 1982 | 17.638394582873 | 1981 | 19.681141015432 | 1980 | 20.505577330638 | 1979 | 20.145718974643 | 1978 | 20.272158901132 | 1977 | 20.90648695971 |
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