| Year | Value | | 2022 | 53.385613029709 | | 2021 | 53.235555644364 | | 2020 | 53.08635829633 | | 2019 | 52.942693352905 | | 2018 | 52.814087702856 | | 2017 | 52.685786099302 | | 2016 | 52.56558114078 | | 2015 | 52.458585838935 | | 2014 | 52.370312333749 | | 2013 | 52.306406042784 | | 2012 | 52.257738157732 | | 2011 | 52.219378081153 | | 2010 | 52.197998907124 | | 2009 | 52.165459719913 | | 2008 | 52.129937454218 | | 2007 | 52.121456805656 | | 2006 | 52.129297557129 | | 2005 | 52.147331448803 | | 2004 | 52.175786702641 | | 2003 | 52.211803743196 | | 2002 | 52.245996750986 | | 2001 | 52.274873618132 | | 2000 | 52.24014419744 | | 1999 | 52.143711686312 | | 1998 | 52.045514092151 | | 1997 | 51.945928061634 | | 1996 | 51.850789918858 | | 1995 | 51.755369536219 | | 1994 | 51.666488050584 | | 1993 | 51.589912280702 | | 1992 | 51.513271285764 | | 1991 | 51.427184850646 | | 1990 | 51.414808561355 | | 1989 | 51.492381755918 | | 1988 | 51.583392673471 | | 1987 | 51.677938237697 | | 1986 | 51.769103967974 | | 1985 | 51.847672501766 | | 1984 | 51.909901917295 | | 1983 | 51.962841769193 | | 1982 | 52.009094750397 | | 1981 | 52.041242575216 | | 1980 | 52.074710264901 | | 1979 | 52.00186763161 | | 1978 | 51.805325463263 | | 1977 | 51.603664219308 | | 1976 | 51.40003777505 | | 1975 | 51.18719889883 | | 1974 | 50.970637261187 | | 1973 | 50.752696828208 | | 1972 | 50.506389821423 | | 1971 | 50.227228703828 | | 1970 | 50.075654887621 | | 1969 | 50.027494959258 | | 1968 | 49.906380272734 | | 1967 | 49.765788137028 | | 1966 | 49.619718309859 | | 1965 | 49.463085798241 | | 1964 | 49.329590795212 | | 1963 | 49.216363229925 | | 1962 | 49.090570157919 | | 1961 | 48.948511343505 | | 1960 | 49.624174553422 |
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