| Year | Value | | 2014 | -36.939563134741 | | 2013 | -50.809061947189 | | 2012 | -50.745794045775 | | 2011 | -50.858420768307 | | 2010 | -48.094799820709 | | 2009 | -59.236911519805 | | 2008 | -66.463537910699 | | 2007 | -68.863994521263 | | 2006 | -70.834772221104 | | 2005 | -66.230103157355 | | 2004 | -65.564577891081 | | 2003 | -67.005745854013 | | 2002 | -61.94284402605 | | 2001 | -57.083488855116 | | 2000 | -77.553506782572 | | 1999 | -63.858776563616 | | 1998 | -59.2251976481 | | 1997 | -60.511649874416 | | 1996 | -59.043230736104 | | 1995 | -53.500180099949 | | 1994 | -50.093424004405 | | 1993 | -46.978235122863 | | 1992 | -43.082590575891 | | 1991 | -53.711454504817 | | 1990 | -48.984782972342 | | 1989 | -36.145178501889 | | 1988 | -32.119669211091 | | 1987 | -33.635323123185 | | 1986 | -16.952878971565 | | 1985 | 10.941526632191 | | 1984 | 21.497654419988 | | 1983 | 20.016297779773 | | 1982 | 22.224666097092 | | 1981 | 21.952918802641 | | 1980 | 22.992681297037 | | 1979 | 23.946996045177 | | 1978 | 25.534821573648 | | 1977 | 22.197718881083 | | 1976 | 16.209813313392 | | 1975 | 15.222633060823 | | 1974 | 13.751155535588 | | 1973 | 16.217644646784 | | 1972 | 15.46637336084 | | 1971 | 14.612364977734 |
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