| Year | Value | | 2019 | 50.07897111053 | | 2018 | 50.098201703347 | | 2017 | 50.119004666765 | | 2016 | 50.142263590602 | | 2015 | 50.168769517137 | | 2014 | 50.19940527892 | | 2013 | 50.233902545171 | | 2012 | 50.270030980793 | | 2011 | 50.30449234417 | | 2010 | 50.334733664273 | | 2009 | 50.358776184323 | | 2008 | 50.377553419805 | | 2007 | 50.394675680886 | | 2006 | 50.416380011277 | | 2005 | 50.447362846165 | | 2004 | 50.49043206822 | | 2003 | 50.544693769982 | | 2002 | 50.606587712805 | | 2001 | 50.669998903448 | | 2000 | 50.729906701859 | | 1999 | 50.783916572706 | | 1998 | 50.833032599078 | | 1997 | 50.87959457747 | | 1996 | 50.928166645663 | | 1995 | 50.98243144756 | | 1994 | 51.043931668654 | | 1993 | 51.111592841487 | | 1992 | 51.183135917386 | | 1991 | 51.253870281674 | | 1990 | 51.319767905483 | | 1989 | 51.378915440185 | | 1988 | 51.431398095036 | | 1987 | 51.477709556802 | | 1986 | 51.519626273469 | | 1985 | 51.558349160524 | | 1984 | 51.594187767136 | | 1983 | 51.626360787536 | | 1982 | 51.655401853036 | | 1981 | 51.681619925625 | | 1980 | 51.705508585719 | | 1979 | 51.727433239953 | | 1978 | 51.748202931213 | | 1977 | 51.767685225772 | | 1976 | 51.786270726097 | | 1975 | 51.804455943601 | | 1974 | 51.822120321402 | | 1973 | 51.84012106571 | | 1972 | 51.859777504195 | | 1971 | 51.883187892124 | | 1970 | 51.911625642353 | | 1969 | 51.946410950177 | | 1968 | 51.987435108619 | | 1967 | 52.034067095918 | | 1966 | 52.083936858329 | | 1965 | 52.136428096298 | | 1964 | 52.19088764293 | | 1963 | 52.248252364263 | | 1962 | 52.307985506395 | | 1961 | 52.369493058217 | | 1960 | 52.432846126672 |
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