| Year | Value | | 2021 | 3.9209793598157 | | 2020 | 3.9707178174755 | | 2019 | 4.0211806570985 | | 2018 | 4.0633061112389 | | 2017 | 4.1058226551739 | | 2016 | 4.1630949494546 | | 2015 | 4.2117628747871 | | 2014 | 4.2616495763879 | | 2013 | 4.3182112965743 | | 2012 | 4.3699575760697 | | 2011 | 4.4354093877337 | | 2010 | 4.5029697672851 | | 2009 | 4.5710292272561 | | 2008 | 4.6304934094238 | | 2007 | 4.6829787933909 | | 2006 | 4.7244972705209 | | 2005 | 4.7793756361117 | | 2004 | 4.8444499725072 | | 2003 | 4.9088259900668 | | 2002 | 4.9781999986111 | | 2001 | 5.0324288923342 | | 2000 | 5.0924363614944 | | 1999 | 5.1553050916903 | | 1998 | 5.2175066232653 | | 1997 | 5.2996627129285 | | 1996 | 5.3733213135224 | | 1995 | 5.4632882881902 | | 1994 | 5.541708033652 | | 1993 | 5.6197104763685 | | 1992 | 5.7050326476336 | | 1991 | 5.793662693751 | | 1990 | 5.876388027856 | | 1989 | 5.9586645745963 | | 1988 | 6.0296238840801 | | 1987 | 6.1148524484393 | | 1986 | 6.1952196451182 | | 1985 | 6.2697367317682 | | 1984 | 6.3383405321494 | | 1983 | 6.3927237836726 | | 1982 | 6.4485253282214 | | 1981 | 6.4958780395191 | | 1980 | 6.5344141425417 | | 1979 | 6.5636518923331 | | 1978 | 6.5807509642497 | | 1977 | 6.5921668273649 | | 1976 | 6.6153897017349 | | 1975 | 6.6364653334017 | | 1974 | 6.6537972824032 | | 1973 | 6.6697035247292 | | 1972 | 6.6868772902464 | | 1971 | 6.6894034332856 | | 1970 | 6.6844904505448 | | 1969 | 6.6790036635424 | | 1968 | 6.6782456715281 | | 1967 | 6.679400312266 | | 1966 | 6.6706920480504 | | 1965 | 6.669869207018 | | 1964 | 6.665797353626 | | 1963 | 6.6575434542557 | | 1962 | 6.655348757652 | | 1961 | 6.637679300092 | | 1960 | 6.6229037265716 |
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