| Year | Value | | 2021 | 4.5990860812647 | | 2020 | 4.5966160306286 | | 2019 | 4.5916759293566 | | 2018 | 4.5867358280845 | | 2017 | 4.5867358280845 | | 2016 | 4.5892058787205 | | 2015 | 4.5916759293566 | | 2014 | 4.5746325799679 | | 2013 | 4.5657403976781 | | 2012 | 4.5447449672718 | | 2011 | 4.5632703470421 | | 2010 | 4.5603062862789 | | 2009 | 4.6161294306533 | | 2008 | 4.5864888230209 | | 2007 | 4.6215141410399 | | 2006 | 4.6360627392862 | | 2005 | 4.6380140792886 | | 2004 | 4.6752211930345 | | 2003 | 4.701165122885 | | 2002 | 4.6846980363097 | | 2001 | 4.7220780536001 | | 2000 | 4.7310527355811 | | 1999 | 4.7314807953563 | | 1998 | 4.7181259725824 | | 1997 | 4.7428101766086 | | 1996 | 4.7647770779301 | | 1995 | 4.7674775842905 | | 1994 | 4.7798115351365 | | 1993 | 4.7914042237866 | | 1992 | 4.703700876868 | | 1991 | 4.6854061998271 | | 1990 | 4.6646412251451 | | 1989 | 4.636466098555 | | 1988 | 4.6154543658145 | | 1987 | 4.6043225886131 | | 1986 | 4.5738750154378 | | 1985 | 4.5534560948499 | | 1984 | 4.5406940842287 | | 1983 | 4.5254620229715 | | 1982 | 4.7022148944053 | | 1981 | 4.8641472150179 | | 1980 | 4.9863117203903 | | 1979 | 5.1073410875685 | | 1978 | 5.2420728996888 | | 1977 | 5.3451950906307 | | 1976 | 5.4557260334864 | | 1975 | 5.5711957327011 | | 1974 | 5.5753458498024 | | 1973 | 5.570652173913 | | 1972 | 5.5360671936759 | | 1971 | 5.5335968379447 | | 1970 | 5.5088932806324 | | 1969 | 5.526185770751 | | 1968 | 5.5187747035573 | | 1967 | 5.5138339920949 | | 1966 | 5.5138339920949 | | 1965 | 5.4792490118577 | | 1964 | 5.5410079051383 | | 1963 | 5.5335968379447 | | 1962 | 5.526185770751 | | 1961 | 5.5187747035573 |
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