| Year | Value | | 2014 | 39.566915144333 | | 2013 | 44.439220886902 | | 2012 | 46.792122976426 | | 2011 | 31.745036723784 | | 2010 | 24.789492506258 | | 2009 | 36.174400897454 | | 2008 | 23.915832145113 | | 2007 | 25.68776877951 | | 2006 | 37.987163792017 | | 2005 | 37.643082123377 | | 2004 | 28.566681273614 | | 2003 | 29.050485999879 | | 2002 | 30.941358505745 | | 2001 | 35.597942039501 | | 2000 | 36.243043697987 | | 1999 | 39.712796084172 | | 1998 | 37.464817806854 | | 1997 | 42.702799130581 | | 1996 | 34.360312012224 | | 1995 | 30.679206158497 | | 1994 | 18.968511069395 | | 1993 | 28.99560845855 | | 1992 | 40.939357016173 | | 1991 | 45.711503802334 | | 1990 | 59.780536121667 | | 1989 | 57.634245413692 | | 1988 | 55.229836632551 | | 1987 | 53.777079256198 | | 1986 | 55.057985784373 | | 1985 | 51.716913464154 | | 1984 | 50.490757086844 | | 1983 | 50.018848769511 | | 1982 | 48.828756360499 | | 1981 | 48.26223825331 | | 1980 | 41.263213968714 | | 1979 | 35.943785364666 | | 1978 | 34.446761976743 | | 1977 | 30.429238215291 | | 1976 | 26.807895767838 | | 1975 | 29.465944393275 | | 1974 | 22.859859914778 | | 1973 | 20.037605931131 | | 1972 | 17.102410677509 | | 1971 | 12.378255140175 |
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