| Year | Value | | 2021 | 29.008936825886 | | 2020 | 28.984694422188 | | 2019 | 28.960451956857 | | 2018 | 28.915254237288 | | 2017 | 28.619414483821 | | 2016 | 28.214101525087 | | 2015 | 28.214101525087 | | 2014 | 28.214101525087 | | 2013 | 28.214101525087 | | 2012 | 28.393256219455 | | 2011 | 28.483386134237 | | 2010 | 28.573516049019 | | 2009 | 28.663645963801 | | 2008 | 28.662909207004 | | 2007 | 29.51509123505 | | 2006 | 29.37940519168 | | 2005 | 29.243719148309 | | 2004 | 39.592082320293 | | 2003 | 39.456396276922 | | 2002 | 39.320710233552 | | 2001 | 39.185024190182 | | 2000 | 38.608511996856 | | 1999 | 39.101525086569 | | 1998 | 39.776885483435 | | 1997 | 39.190549866156 | | 1996 | 39.555858444461 | | 1995 | 39.421400329085 | | 1994 | 39.39008816523 | | 1993 | 39.387018345244 | | 1992 | 39.162921486284 | | 1991 | 38.677889928535 | | 1990 | 37.758785824799 | | 1989 | 37.881578624229 | | 1988 | 37.986566467742 | | 1987 | 37.470222746138 | | 1986 | 37.267614627078 | | 1985 | 37.065006508018 | | 1984 | 36.555416390383 | | 1983 | 36.352808271323 | | 1982 | 36.1422186203 | | 1981 | 35.781821753972 | | 1980 | 35.433704167588 | | 1979 | 36.781355141335 | | 1978 | 36.1422186203 | | 1977 | 36.649352881947 | | 1976 | 36.807141629215 | | 1975 | 37.107983987819 | | 1974 | 37.427245266338 | | 1973 | 37.747120508853 | | 1972 | 37.329011026793 | | 1971 | 36.932390284634 | | 1970 | 36.653650629927 | | 1969 | 36.635231710012 | | 1968 | 36.592254230212 | | 1967 | 36.561556030354 | | 1966 | 36.518578550554 | | 1965 | 36.512438910582 | | 1964 | 36.475601070753 | | 1963 | 36.438763230924 | | 1962 | 36.401925391095 | | 1961 | 36.390260075149 |
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