| Year | Value | | 2022 | 50.992249287256 | | 2021 | 50.993760513331 | | 2020 | 50.998042300163 | | 2019 | 50.999189339898 | | 2018 | 50.998577479801 | | 2017 | 50.998322923663 | | 2016 | 50.998642780031 | | 2015 | 50.999947753093 | | 2014 | 51.030865187411 | | 2013 | 51.094438059326 | | 2012 | 51.164215470837 | | 2011 | 51.240817412056 | | 2010 | 51.294917895131 | | 2009 | 51.298121140455 | | 2008 | 51.279293167516 | | 2007 | 51.263241125331 | | 2006 | 51.249492239816 | | 2005 | 51.239176612714 | | 2004 | 51.242456307631 | | 2003 | 51.267965223622 | | 2002 | 51.303978831163 | | 2001 | 51.342607679051 | | 2000 | 51.400109294595 | | 1999 | 51.485087965255 | | 1998 | 51.56808029843 | | 1997 | 51.640795360358 | | 1996 | 51.715269064527 | | 1995 | 51.791813248115 | | 1994 | 51.872285857654 | | 1993 | 51.958489849455 | | 1992 | 52.052366643061 | | 1991 | 52.154195598294 | | 1990 | 52.080184037588 | | 1989 | 51.828289860558 | | 1988 | 51.661016480095 | | 1987 | 51.577790274132 | | 1986 | 51.503492978034 | | 1985 | 51.441778366801 | | 1984 | 51.397658231713 | | 1983 | 51.376182798621 | | 1982 | 51.381748464385 | | 1981 | 51.418405852907 | | 1980 | 51.35856025312 | | 1979 | 51.153181235807 | | 1978 | 50.917439231699 | | 1977 | 50.682697462329 | | 1976 | 50.451406544739 | | 1975 | 50.228618656749 | | 1974 | 50.122831040206 | | 1973 | 50.12952990831 | | 1972 | 50.135710975691 | | 1971 | 50.143185691006 | | 1970 | 50.15170905074 | | 1969 | 50.16095780921 | | 1968 | 50.170859760329 | | 1967 | 50.181619497634 | | 1966 | 50.193240177684 | | 1965 | 50.206413295578 | | 1964 | 50.221382549045 | | 1963 | 50.23813062875 | | 1962 | 50.255729076148 | | 1961 | 50.274277211242 | | 1960 | 50.294404500473 |
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