| Year | Value | | 2014 | 7.9746993778389 | | 2013 | 6.9071855089162 | | 2012 | 6.4703917591118 | | 2011 | 4.1597585045913 | | 2010 | 4.8956701288712 | | 2009 | 3.8623237865356 | | 2008 | 2.4996675905671 | | 2007 | 2.2907260259855 | | 2006 | 1.3261736385097 | | 2005 | 0.62569978633591 | | 2004 | 0.025327812985795 | | 2003 | -1.5714331768063 | | 2002 | -3.1190858752351 | | 2001 | -5.1538114782998 | | 2000 | -4.5415703604728 | | 1999 | -4.2469206653439 | | 1998 | -6.0905733310796 | | 1997 | -5.5645295054094 | | 1996 | -6.4120888500157 | | 1995 | -6.6524418239737 | | 1994 | -7.2287608938877 | | 1993 | -6.017642887882 | | 1992 | -9.1283814254835 | | 1991 | -10.854389979787 | | 1990 | -7.2646103818925 | | 1989 | -11.790587534936 | | 1988 | -8.9764634719034 | | 1987 | -10.412552162677 | | 1986 | -12.951904760745 | | 1985 | -13.5053206719 | | 1984 | -13.555826230851 | | 1983 | -10.209577162911 | | 1982 | -8.8960737653751 | | 1981 | -9.6598065205661 | | 1980 | -9.5900195606049 | | 1979 | -9.8118626415829 | | 1978 | -9.2404070697441 | | 1977 | -9.8402443900991 | | 1976 | -9.5292028932815 | | 1975 | -8.0374331234094 | | 1974 | -8.5926432968935 | | 1973 | -7.846461563904 | | 1972 | -5.9994645287242 | | 1971 | -4.3990989245061 |
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