| Year | Value | | 2019 | 21.719529618728 | | 2018 | 19.717145849921 | | 2017 | 17.399154474548 | | 2016 | 17.40228938726 | | 2015 | 13.862030055917 | | 2014 | 18.602178781295 | | 2013 | 16.283155850738 | | 2012 | 13.533167603276 | | 2011 | 12.125648930762 | | 2010 | 11.387262545418 | | 2009 | 11.46153310294 | | 2008 | 12.651905417944 | | 2007 | 14.049542570394 | | 2006 | 12.468137561402 | | 2005 | 12.19192579885 | | 2004 | 13.259582078317 | | 2003 | 11.369185855802 | | 2002 | 13.631081215314 | | 2001 | 13.282810181298 | | 2000 | 12.691753054564 | | 1999 | 12.457608046094 | | 1998 | 15.798575388742 | | 1997 | 16.187851335116 | | 1996 | 15.243891144747 | | 1995 | 15.897111247321 | | 1994 | 14.50610152145 | | 1993 | 9.6853137699726 | | 1992 | 9.5752064164883 | | 1991 | 10.756541161898 | | 1990 | 9.6883739569081 | | 1989 | 4.5142046456483 | | 1988 | 0.77043228053151 | | 1987 | 4.3578638136269 | | 1986 | 4.1373739525572 | | 1985 | 2.6342242050719 | | 1984 | 7.5166140214056 | | 1983 | 8.1925154980758 | | 1982 | 13.221547512954 | | 1981 | -3.2515576185361 | | 1980 | 4.1724328835441 | | 1979 | 12.641384876782 | | 1978 | 10.504056432572 | | 1977 | 13.615101292989 | | 1976 | 11.215815138723 | | 1975 | 10.149334631432 | | 1974 | 11.101891650631 |
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