| Year | Value | | 2015 | 55.52708265301 | | 2014 | -44.916192885495 | | 2013 | -44.726138498955 | | 2012 | -39.687809719133 | | 2011 | -39.37928924259 | | 2010 | -41.759477189813 | | 2009 | -42.842516932366 | | 2008 | -39.85107268896 | | 2007 | -38.804704017294 | | 2006 | -37.943281122877 | | 2005 | -38.861888245744 | | 2004 | -36.139194038756 | | 2003 | -32.299254895349 | | 2002 | -29.641146162574 | | 2001 | -26.609145498418 | | 2000 | -23.598222967055 | | 1999 | -21.111596770429 | | 1998 | -19.105110496837 | | 1997 | -16.867743005606 | | 1996 | -16.075899305548 | | 1995 | -15.283335926305 | | 1994 | -15.781257754129 | | 1993 | -11.301925072608 | | 1992 | -10.972335469475 | | 1991 | -8.4618742120923 | | 1990 | -12.186782588677 | | 1989 | 23.505544333292 | | 1988 | 21.067172577658 | | 1987 | 21.270329225648 | | 1986 | 18.711965088496 | | 1985 | 17.137359530067 | | 1984 | 14.590736420822 | | 1983 | 16.38496333748 | | 1982 | 17.169015110428 | | 1981 | 21.231962887379 | | 1980 | 18.530062983903 | | 1979 | 13.919281420482 | | 1978 | 13.744811082978 | | 1977 | 12.477997669561 | | 1976 | 10.135460775003 | | 1975 | 7.6602367627663 | | 1974 | 5.9081711407162 | | 1973 | 5.1528995263531 | | 1972 | 2.6758964154464 | | 1971 | 2.68530957512 | | 1970 | -8.1242822755628 | | 1969 | -9.1343594327208 | | 1968 | -9.8337016679542 | | 1967 | -9.9973017439802 | | 1966 | -10.977536992129 | | 1965 | -11.365583137978 | | 1964 | -13.024583773281 | | 1963 | -11.124620546235 | | 1962 | -9.3189938288305 | | 1961 | -12.839164755051 | | 1960 | -14.359811407812 |
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