| Year | Value | | 2015 | -4.6664853127365 | | 2014 | -10.29714478569 | | 2013 | -10.254601829366 | | 2012 | -13.076599896953 | | 2011 | -15.639039197804 | | 2010 | -14.86782343892 | | 2009 | -17.281177324468 | | 2008 | -15.817578974287 | | 2007 | -19.295772710674 | | 2006 | -22.332610579965 | | 2005 | -23.064135675774 | | 2004 | -23.830322174719 | | 2003 | -25.487129804318 | | 2002 | -22.616241891906 | | 2001 | -20.907716690254 | | 2000 | -19.457257624189 | | 1999 | -18.232514610761 | | 1998 | -17.837997862089 | | 1997 | -17.768767986131 | | 1996 | -16.981663180088 | | 1995 | -14.89229877711 | | 1994 | -13.166264963225 | | 1993 | -14.560123770523 | | 1992 | -14.144161073293 | | 1991 | -14.257666038416 | | 1990 | -14.503921943821 | | 1989 | -12.869089742816 | | 1988 | -13.311703066671 | | 1987 | -13.252053577433 | | 1986 | -14.640748147171 | | 1985 | -17.667856577614 | | 1984 | -18.426840246463 | | 1983 | -17.222271445256 | | 1982 | -13.145897873849 | | 1981 | -6.2315197739254 | | 1980 | 2.2157498917391 | | 1979 | 9.9593176265494 | | 1978 | 14.041913310038 | | 1977 | 17.592942673398 | | 1976 | 19.504698463756 | | 1975 | 18.53132093742 | | 1974 | 18.569896339808 | | 1973 | 18.347991248747 | | 1972 | 16.835634723025 | | 1971 | 15.939317031004 |
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