| Year | Value | | 2021 | 4.0494248832187 | | 2020 | 3.989150635391 | | 2019 | 3.9399266663318 | | 2018 | 3.8806569893013 | | 2017 | 3.8545068059672 | | 2016 | 3.7508316841629 | | 2015 | 3.8071998593601 | | 2014 | 3.732171379778 | | 2013 | 3.7480235069566 | | 2012 | 3.7128936661811 | | 2011 | 3.6365462855995 | | 2010 | 3.6877793962529 | | 2009 | 3.7058616706012 | | 2008 | 3.5581897634236 | | 2007 | 3.5541714802351 | | 2006 | 3.5491486262494 | | 2005 | 3.5391029182782 | | 2004 | 3.4938972324075 | | 2003 | 3.4245818474057 | | 2002 | 3.4396504093626 | | 2001 | 3.3532573208097 | | 2000 | 3.3060424933447 | | 1999 | 3.4989200863931 | | 1998 | 3.2745190617309 | | 1997 | 3.2597518710131 | | 1996 | 3.1909789542418 | | 1995 | 3.2980059269677 | | 1994 | 3.0264302576724 | | 1993 | 3.0289617760812 | | 1992 | 3.0099753880155 | | 1991 | 2.6550806168065 | | 1990 | 2.6601034707921 | | 1989 | 2.5827515194133 | | 1988 | 2.5927972273846 | | 1987 | 2.5586418202823 | | 1986 | 2.5787332362248 | | 1985 | 2.5084132804259 | | 1984 | 2.4694962077452 | | 1983 | 2.4458687025968 | | 1982 | 2.4564166959667 | | 1981 | 2.4792807273093 | | 1980 | 2.4561755989753 | | 1979 | 2.4581847405696 | | 1978 | 2.5516098247024 | | 1977 | 2.6470440504295 | | 1976 | 2.7424782761565 | | 1975 | 2.8379125018836 | | 1974 | 2.8559947762319 | | 1973 | 2.8680496257974 | | 1972 | 2.8680496257974 | | 1971 | 2.865035913406 | | 1970 | 2.8559947762319 | | 1969 | 2.8479582098548 | | 1968 | 2.8138028027525 | | 1967 | 2.8138028027525 | | 1966 | 2.7927068160129 | | 1965 | 2.6842131699231 | | 1964 | 2.5174544176001 | | 1963 | 2.5013812848461 | | 1962 | 2.516449846803 | | 1961 | 2.5797378070219 |
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