| Year | Value | | 2014 | -19.046948392993 | | 2013 | -19.754464678705 | | 2012 | -21.943563637163 | | 2011 | -27.495387784307 | | 2010 | -29.565927820455 | | 2009 | -25.732776606188 | | 2008 | -26.216246990705 | | 2007 | -28.748093993832 | | 2006 | -30.727051969625 | | 2005 | -32.258744574968 | | 2004 | -33.265218126205 | | 2003 | -33.530248141061 | | 2002 | -29.747144229048 | | 2001 | -34.597313227177 | | 2000 | -33.972145201316 | | 1999 | -29.739958988683 | | 1998 | -31.328665139735 | | 1997 | -34.118393388562 | | 1996 | -32.342202250541 | | 1995 | -31.842297892005 | | 1994 | -31.481745442668 | | 1993 | -27.920566927085 | | 1992 | -26.537673098249 | | 1991 | -24.845154994741 | | 1990 | -23.104335227139 | | 1989 | -34.875366881684 | | 1988 | -29.157807169814 | | 1987 | -24.227173562202 | | 1986 | -27.47161317412 | | 1985 | -29.164108245937 | | 1984 | -27.597580911943 | | 1983 | -23.139906537963 | | 1982 | -24.930223043014 | | 1981 | -30.649346335558 | | 1980 | -47.723918001767 | | 1979 | -55.119506032415 | | 1978 | -46.194873279236 | | 1977 | -53.419241520043 | | 1976 | -56.491516543357 | | 1975 | -49.204430685858 | | 1974 | -64.452544386766 | | 1973 | -59.820797372091 | | 1972 | -54.30289862357 | | 1971 | -44.51670993972 |
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