| Year | Value | | 2015 | 28.008175404145 | | 2014 | -8.1057035413166 | | 2013 | -9.800515395279 | | 2012 | -10.700146334821 | | 2011 | -12.932638134924 | | 2010 | -14.692633705246 | | 2009 | -16.001393251433 | | 2008 | -18.150064802092 | | 2007 | -19.22930786747 | | 2006 | -20.693832937898 | | 2005 | -21.858430714181 | | 2004 | -22.127607107202 | | 2003 | -22.796154972059 | | 2002 | -22.568881864185 | | 2001 | -23.498949526338 | | 2000 | -23.424919096018 | | 1999 | -22.308299828108 | | 1998 | -23.196388959067 | | 1997 | -21.398889400499 | | 1996 | -20.73189561604 | | 1995 | -20.223410942844 | | 1994 | -20.698073225153 | | 1993 | -19.32083534758 | | 1992 | -20.281605976677 | | 1991 | -19.549270469352 | | 1990 | -22.315861914148 | | 1989 | -26.778042070401 | | 1988 | -24.675579161014 | | 1987 | -23.529232120595 | | 1986 | -23.585161823641 | | 1985 | -25.567283161834 | | 1984 | -25.467787586807 | | 1983 | -23.624417284036 | | 1982 | -21.740197513126 | | 1981 | -19.432205762854 | | 1980 | -27.79320158172 | | 1979 | -37.607223533666 | | 1978 | -40.173081844097 | | 1977 | -43.078071324497 | | 1976 | -45.946504017903 | | 1975 | -41.938446224355 | | 1974 | -48.56326727036 | | 1973 | -52.372647769429 | | 1972 | -48.148279803473 | | 1971 | -48.388798556693 | | 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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