| Year | Value | | 2021 | 1.878893417315 | | 2020 | 1.878893417315 | | 2019 | 1.8690132147709 | | 2018 | 1.8640731134988 | | 2017 | 1.8640731134988 | | 2016 | 1.8665431641349 | | 2015 | 1.8690132147709 | | 2014 | 1.8467827590466 | | 2013 | 1.8356675311844 | | 2012 | 1.8171421514141 | | 2011 | 1.8270223539583 | | 2010 | 1.8183771767321 | | 2009 | 1.8220822526862 | | 2008 | 1.7825614425096 | | 2007 | 1.7924169445474 | | 2006 | 1.8062986291219 | | 2005 | 1.8060269235519 | | 2004 | 1.83310682969 | | 2003 | 1.8507266888971 | | 2002 | 1.8214153390145 | | 2001 | 1.8489151537607 | | 2000 | 1.8517147091515 | | 1999 | 1.8497962208225 | | 1998 | 1.8479271335062 | | 1997 | 1.8556914906756 | | 1996 | 1.8735828084476 | | 1995 | 1.876900827467 | | 1994 | 1.8812071137458 | | 1993 | 1.9144127454613 | | 1992 | 1.8063314807954 | | 1991 | 1.7725989872792 | | 1990 | 1.7388662467581 | | 1989 | 1.7372441645054 | | 1988 | 1.7156149191058 | | 1987 | 1.7088057305175 | | 1986 | 1.689102877609 | | 1985 | 1.6770821291836 | | 1984 | 1.6828454983327 | | 1983 | 1.6762586143016 | | 1982 | 1.7094397925158 | | 1981 | 1.7450907743609 | | 1980 | 1.7508541435099 | | 1979 | 1.7463493357041 | | 1978 | 1.7647058823529 | | 1977 | 1.7625655652689 | | 1976 | 1.7530169901714 | | 1975 | 1.7484071714328 | | 1974 | 1.7546936758893 | | 1973 | 1.7514822134387 | | 1972 | 1.7193675889328 | | 1971 | 1.7243083003953 | | 1970 | 1.7070158102767 | | 1969 | 1.7193675889328 | | 1968 | 1.721837944664 | | 1967 | 1.721837944664 | | 1966 | 1.7119565217391 | | 1965 | 1.7020750988142 | | 1964 | 1.7440711462451 | | 1963 | 1.7366600790514 | | 1962 | 1.7341897233202 | | 1961 | 1.7317193675889 |
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