| Year | Value | | 2021 | 1.5454778748947 | | 2020 | 1.5337379245456 | | 2019 | 1.5770539879721 | | 2018 | 1.6057562983318 | | 2017 | 1.6329842364073 | | 2016 | 1.665588056713 | | 2015 | 1.6702898812345 | | 2014 | 1.6725301694364 | | 2013 | 1.6599995531933 | | 2012 | 1.6929556853445 | | 2011 | 1.6853244081583 | | 2010 | 1.7034508282157 | | 2009 | 1.716883091812 | | 2008 | 1.7435375766895 | | 2007 | 1.7343720436356 | | 2006 | 1.7082134124493 | | 2005 | 1.668048048438 | | 2004 | 1.6692412547606 | | 2003 | 1.6609010750483 | | 2002 | 1.6534788216838 | | 2001 | 1.6694838424818 | | 2000 | 1.7062886241123 | | 1999 | 1.6757234080508 | | 1998 | 1.6804747058166 | | 1997 | 1.6932941370527 | | 1996 | 1.7036111109525 | | 1995 | 1.7107917658344 | | 1994 | 1.7477275300687 | | 1993 | 1.767058138332 | | 1992 | 1.8029025009712 | | 1991 | 1.8247748020288 | | 1990 | 1.8517277947721 | | 1989 | 1.8368036954515 | | 1988 | 1.8451108642495 | | 1987 | 1.8257106433948 | | 1986 | 1.8311004510838 | | 1985 | 1.8533769118698 | | 1984 | 1.8566872972722 | | 1983 | 1.8812671900905 | | 1982 | 1.9229859060576 | | 1981 | 1.9380455904318 | | 1980 | 1.9831782129732 | | 1979 | 1.9952278813144 | | 1978 | 1.9867899912988 | | 1977 | 2.0287176044434 | | 1976 | 2.0637079226665 | | 1975 | 2.1160186258452 | | 1974 | 2.2069277513202 | | 1973 | 2.253452454988 | | 1972 | 2.3394553688243 | | 1971 | 2.4795816442029 | | 1970 | 2.5481723553398 | | 1969 | 2.589033234759 | | 1968 | 2.6376033117281 | | 1967 | 2.7173734204436 | | 1966 | 2.668134030673 | | 1965 | 2.8301618143083 | | 1964 | 2.9269059547413 | | 1963 | 2.9486537324998 | | 1962 | 2.9697533307965 | | 1961 | 3.0126592170648 | | 1960 | 3.0215802153097 |
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