| Year | Value | | 2018 | 50.441295556287 | | 2017 | 50.439494280554 | | 2016 | 50.43684176788 | | 2015 | 50.433669695859 | | 2014 | 50.429028562399 | | 2013 | 50.423392235584 | | 2012 | 50.416997644588 | | 2011 | 50.410287328504 | | 2010 | 50.402931146476 | | 2009 | 50.394875016896 | | 2008 | 50.387004794386 | | 2007 | 50.378103063476 | | 2006 | 50.368997504897 | | 2005 | 50.359615777642 | | 2004 | 50.350116370578 | | 2003 | 50.340784584706 | | 2002 | 50.331179321486 | | 2001 | 50.319934520008 | | 2000 | 50.308652218645 | | 1999 | 50.294883455327 | | 1998 | 50.280956649631 | | 1997 | 50.264864380273 | | 1996 | 50.248963929394 | | 1995 | 50.232228425264 | | 1994 | 50.214804880159 | | 1993 | 50.197208805395 | | 1992 | 50.178942110738 | | 1991 | 50.161722968908 | | 1990 | 50.142614881511 | | 1989 | 50.123862095196 | | 1988 | 50.104391129074 | | 1987 | 50.084561937305 | | 1986 | 50.063292004429 | | 1985 | 50.045449812432 | | 1984 | 50.025618387344 | | 1983 | 50.005058199043 | | 1982 | 49.98654203778 | | 1981 | 49.967728427634 | | 1980 | 49.952733805237 | | 1979 | 49.938309166888 | | 1978 | 49.923943229552 | | 1977 | 49.911381898711 | | 1976 | 49.898480211161 | | 1975 | 49.890199584119 | | 1974 | 49.880251252464 | | 1973 | 49.873672744844 | | 1972 | 49.865191929072 | | 1971 | 49.857870399966 | | 1970 | 49.84894916433 | | 1969 | 49.840230068701 | | 1968 | 49.828175578264 | | 1967 | 49.814729041223 | | 1966 | 49.802106139202 | | 1965 | 49.788120272479 | | 1964 | 49.779644879811 | | 1963 | 49.769178174486 | | 1962 | 49.759637326193 | | 1961 | 49.75147953376 | | 1960 | 49.746235388914 |
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