| Year | Value | | 2021 | 1.8532845922369 | | 2020 | 1.8840757634943 | | 2019 | 1.9237639482941 | | 2018 | 1.9727948410443 | | 2017 | 1.9990754531857 | | 2016 | 2.0187126222717 | | 2015 | 2.0703731549402 | | 2014 | 2.0939027349547 | | 2013 | 2.1117366250886 | | 2012 | 2.1376591405373 | | 2011 | 2.1702289756511 | | 2010 | 2.1892516842779 | | 2009 | 2.2124230553208 | | 2008 | 2.2336992917609 | | 2007 | 2.2661586280951 | | 2006 | 2.3060759418253 | | 2005 | 2.3492882969377 | | 2004 | 2.3860560584594 | | 2003 | 2.4226251810512 | | 2002 | 2.4840622930214 | | 2001 | 2.5456374228398 | | 2000 | 2.6096850999393 | | 1999 | 2.6728378384073 | | 1998 | 2.7298497491051 | | 1997 | 2.7896216335308 | | 1996 | 2.8526986156044 | | 1995 | 2.9257804677703 | | 1994 | 2.9870936828634 | | 1993 | 3.0509598985301 | | 1992 | 3.1093506286436 | | 1991 | 3.1854279203079 | | 1990 | 3.2623597960557 | | 1989 | 3.340133332012 | | 1988 | 3.4163793990448 | | 1987 | 3.5066179408634 | | 1986 | 3.596474781428 | | 1985 | 3.687818314317 | | 1984 | 3.7792992371967 | | 1983 | 3.8742854224494 | | 1982 | 3.974149380984 | | 1981 | 4.068456127743 | | 1980 | 4.1677668904729 | | 1979 | 4.2747860503673 | | 1978 | 4.382253052636 | | 1977 | 4.4772864946423 | | 1976 | 4.57363357881 | | 1975 | 4.6700172680717 | | 1974 | 4.7768388691257 | | 1973 | 4.8960400172369 | | 1972 | 4.9968423863736 | | 1971 | 5.0984108145371 | | 1970 | 5.1978724769625 | | 1969 | 5.2933939938944 | | 1968 | 5.3863866595351 | | 1967 | 5.4945617370607 | | 1966 | 5.5959661490076 | | 1965 | 5.6845602509268 | | 1964 | 5.7627077353065 | | 1963 | 5.8208499725956 | | 1962 | 5.8451288286165 | | 1961 | 5.8585739673328 | | 1960 | 5.8658885484197 |
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