| Year | Value | | 2015 | 34.629990267526 | | 2014 | 39.674631806083 | | 2013 | 42.323917286428 | | 2012 | 39.180828861754 | | 2011 | 30.841231990666 | | 2010 | 26.570496786812 | | 2009 | 19.119915061584 | | 2008 | 19.823124943992 | | 2007 | 16.55018289399 | | 2006 | 14.7055545574 | | 2005 | 7.8619833315447 | | 2004 | -1.706573507816 | | 2003 | -10.718130373813 | | 2002 | -18.017381995473 | | 2001 | -17.24190753071 | | 2000 | -22.211813179828 | | 1999 | -26.806219914753 | | 1998 | -22.755761828191 | | 1997 | -22.327269359737 | | 1996 | -19.168927806693 | | 1995 | -19.034169617795 | | 1994 | -13.258332177197 | | 1993 | -4.2410482395667 | | 1992 | -1.3182926052157 | | 1991 | 0.84983071333341 | | 1990 | -1.0079999894329 | | 1989 | -1.0008346090176 | | 1988 | -12.795478700504 | | 1987 | -17.857741399021 | | 1986 | -21.320333059706 | | 1985 | -18.613582247114 | | 1984 | -7.9298513559838 | | 1983 | -22.632695924941 | | 1982 | -15.859330937761 | | 1981 | -7.7937245287578 | | 1980 | 0.29408986743548 | | 1979 | 11.091824606514 | | 1978 | 18.945915612763 | | 1977 | 25.496066764558 | | 1976 | 37.734105309566 | | 1975 | 42.437451284734 | | 1974 | 51.094739756953 | | 1973 | 50.238605216557 | | 1972 | 51.924541650361 | | 1971 | 47.378928330182 | | 1970 | 50.595481812807 | | 1969 | 48.350606595476 | | 1968 | 43.248040716237 | | 1967 | 40.59059187059 | | 1966 | 39.841947891783 | | 1965 | 36.600268200155 | | 1964 | 32.80220314794 | | 1963 | 31.5132693172 | | 1962 | 28.215671003381 | | 1961 | 28.575449200234 | | 1960 | 27.761071106614 |
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