| Year | Value | | 2015 | 28.008175404145 | | 2014 | -6.7287038286156 | | 2013 | -7.9251817780635 | | 2012 | -8.9051738500423 | | 2011 | -10.013243075432 | | 2010 | -12.781346637657 | | 2009 | -14.284517208136 | | 2008 | -16.500384069451 | | 2007 | -17.108686496449 | | 2006 | -18.757574974924 | | 2005 | -20.86636455689 | | 2004 | -22.075361676752 | | 2003 | -22.955636996741 | | 2002 | -24.248720270013 | | 2001 | -24.995716247776 | | 2000 | -24.007833505946 | | 1999 | -22.661052930342 | | 1998 | -22.722458242378 | | 1997 | -19.912349959034 | | 1996 | -18.965106013061 | | 1995 | -18.734513958273 | | 1994 | -18.790539074482 | | 1993 | -17.741736521409 | | 1992 | -19.544082824117 | | 1991 | -19.979038436005 | | 1990 | -23.743672891797 | | 1989 | -18.671150482643 | | 1988 | -16.770075075614 | | 1987 | -15.658490891062 | | 1986 | -16.747501539489 | | 1985 | -17.375925572996 | | 1984 | -15.837422777014 | | 1983 | -13.542205385798 | | 1982 | -12.185262639416 | | 1981 | -9.6072412713749 | | 1980 | -18.319780353141 | | 1979 | -21.95513902341 | | 1978 | -15.648313659537 | | 1977 | -15.075676947441 | | 1976 | -15.797469056524 | | 1975 | -13.639669505415 | | 1974 | -12.843718778296 | | 1973 | -17.830343241856 | | 1972 | -16.527791872166 | | 1971 | -23.228000697489 | | 1970 | 23.184991572491 | | 1969 | 19.435914726701 | | 1968 | 18.088274208953 | | 1967 | 16.256700210398 | | 1966 | 19.350595763956 | | 1965 | 18.820090522769 | | 1964 | 19.243570633854 | | 1963 | 18.026198345036 | | 1962 | 17.941880543195 | | 1961 | 12.586700771082 | | 1960 | 12.335653942187 |
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