| Year | Value | | 2015 | -4.6664853127365 | | 2014 | -10.793600982405 | | 2013 | -12.725915036786 | | 2012 | -13.981534706785 | | 2011 | -18.016270668029 | | 2010 | -22.697887892409 | | 2009 | -24.748011212429 | | 2008 | -26.918941825734 | | 2007 | -36.638312780609 | | 2006 | -42.219343381753 | | 2005 | -44.959310798076 | | 2004 | -52.035066556396 | | 2003 | -52.04619803639 | | 2002 | -51.790161492708 | | 2001 | -50.536122146559 | | 2000 | -51.53968913281 | | 1999 | -49.958579944703 | | 1998 | -55.211967431778 | | 1997 | -59.386941139737 | | 1996 | -59.026718949962 | | 1995 | -54.792244037638 | | 1994 | -50.875767636433 | | 1993 | -54.95049285564 | | 1992 | -55.252071447211 | | 1991 | -57.110825677487 | | 1990 | -58.096923469783 | | 1989 | -62.113978760803 | | 1988 | -68.948573544589 | | 1987 | -71.738921424136 | | 1986 | -72.321195885729 | | 1985 | -77.543135169856 | | 1984 | -86.640915531342 | | 1983 | -93.053771428634 | | 1982 | -84.025602772462 | | 1981 | -68.604327496995 | | 1980 | -54.585615370392 | | 1979 | -35.29230294782 | | 1978 | -24.141398776015 | | 1977 | -15.629319402694 | | 1976 | -7.2825820275132 | | 1975 | -6.5471630820393 | | 1974 | 1.2819347551468 | | 1973 | 10.054968371842 | | 1972 | 5.2416386883934 | | 1971 | 0.87988467648872 |
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