| Year | Value | | 2014 | -14.483553291849 | | 2013 | -18.588886274417 | | 2012 | -19.045953511504 | | 2011 | -15.504667324091 | | 2010 | -15.650483248251 | | 2009 | -11.492119081313 | | 2008 | -10.285703674815 | | 2007 | -16.556267409544 | | 2006 | -23.717888674996 | | 2005 | -23.141225584844 | | 2004 | -22.692481382868 | | 2003 | -30.375217859281 | | 2002 | -30.707345607047 | | 2001 | -29.243364103717 | | 2000 | -33.549844744241 | | 1999 | -32.711498398744 | | 1998 | -34.301330033336 | | 1997 | -31.925494938024 | | 1996 | -27.580697522522 | | 1995 | -30.165148176914 | | 1994 | -30.675416230431 | | 1993 | -29.541254361836 | | 1992 | -29.863424100593 | | 1991 | -23.109650431752 | | 1990 | -25.923064348959 | | 1989 | -23.344100174648 | | 1988 | -20.469494845056 | | 1987 | -20.459588489336 | | 1986 | -25.185868794819 | | 1985 | -26.767349449639 | | 1984 | -19.209881574231 | | 1983 | -14.4391374941 | | 1982 | -11.47536165094 | | 1981 | -12.527787987103 | | 1980 | -11.910219542329 | | 1979 | -6.9043744444973 | | 1978 | 2.5892359360732 | | 1977 | 5.1455197510454 | | 1976 | 12.188826117479 | | 1975 | 16.22431473933 | | 1974 | 16.118972764418 | | 1973 | 17.938894622707 | | 1972 | 17.859018227093 | | 1971 | 16.851137182598 |
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