| Year | Value | | 2015 | 28.008175404145 | | 2014 | -8.1578255408319 | | 2013 | -9.9117230009229 | | 2012 | -10.744490652798 | | 2011 | -13.236189691955 | | 2010 | -14.907527158647 | | 2009 | -16.075601905578 | | 2008 | -18.155286870809 | | 2007 | -19.301666299422 | | 2006 | -20.685270181382 | | 2005 | -21.890918802342 | | 2004 | -22.268730174391 | | 2003 | -23.11194191549 | | 2002 | -22.947854246448 | | 2001 | -23.799039270564 | | 2000 | -23.710735954991 | | 1999 | -22.494618991302 | | 1998 | -23.305702019758 | | 1997 | -21.568024511832 | | 1996 | -20.915486333773 | | 1995 | -20.449872266835 | | 1994 | -20.893807559186 | | 1993 | -19.338781152377 | | 1992 | -20.156898048007 | | 1991 | -19.423232140509 | | 1990 | -21.995492016127 | | 1989 | -25.984623811601 | | 1988 | -23.754665718382 | | 1987 | -22.329622656858 | | 1986 | -22.200466708433 | | 1985 | -24.025674134479 | | 1984 | -23.876562030804 | | 1983 | -22.126654793182 | | 1982 | -20.453220924556 | | 1981 | -18.34154590139 | | 1980 | -26.313570506289 | | 1979 | -35.618606693299 | | 1978 | -38.247703929461 | | 1977 | -41.00202494171 | | 1976 | -43.778416210237 | | 1975 | -39.885501360437 | | 1974 | -45.79203664332 | | 1973 | -49.3443305637 | | 1972 | -45.382658246937 | | 1971 | -45.433732302182 | | 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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