| Year | Value | | 2021 | 5.3032928942808 | | 2020 | 5.3032928942808 | | 2019 | 5.6135181975737 | | 2018 | 5.9246100519931 | | 2017 | 6.2357019064125 | | 2016 | 6.5467937608319 | | 2015 | 6.5281629116118 | | 2014 | 6.5095320623917 | | 2013 | 6.4909012131716 | | 2012 | 6.3390381282496 | | 2011 | 6.1871750433276 | | 2010 | 6.0658578856152 | | 2009 | 5.892547660312 | | 2008 | 5.5892547660312 | | 2007 | 5.3726169844021 | | 2006 | 5.1993067590988 | | 2005 | 4.9826689774697 | | 2004 | 4.7876949740035 | | 2003 | 4.5927209705373 | | 2002 | 4.3977469670711 | | 2001 | 4.2027729636048 | | 2000 | 3.9861351819757 | | 1999 | 3.7998266897747 | | 1998 | 3.734835355286 | | 1997 | 3.7001733102253 | | 1996 | 3.5701906412478 | | 1995 | 3.5875216637782 | | 1994 | 3.5615251299827 | | 1993 | 3.4618717504333 | | 1992 | 3.4618717504333 | | 1991 | 3.4618717504333 | | 1990 | 3.4618717504333 | | 1989 | 3.4445407279029 | | 1988 | 3.4445407279029 | | 1987 | 3.4358752166378 | | 1986 | 3.4358752166378 | | 1985 | 3.4315424610052 | | 1984 | 3.42287694974 | | 1983 | 3.4012131715771 | | 1982 | 3.4012131715771 | | 1981 | 3.3795493934142 | | 1980 | 3.3795493934142 | | 1979 | 3.3795493934142 | | 1978 | 3.3578856152513 | | 1977 | 3.3362218370884 | | 1976 | 3.2928942807626 | | 1975 | 3.2495667244367 | | 1974 | 3.2062391681109 | | 1973 | 3.1195840554593 | | 1972 | 3.0329289428076 | | 1971 | 2.9029462738302 | | 1970 | 2.8812824956672 | | 1969 | 2.8596187175043 | | 1968 | 2.8509532062392 | | 1967 | 2.8162911611785 | | 1966 | 2.8162911611785 | | 1965 | 2.7729636048527 | | 1964 | 2.7729636048527 | | 1963 | 2.7729636048527 | | 1962 | 2.7729636048527 | | 1961 | 2.7729636048527 |
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