| Year | Value | | 2021 | 3.9856876966704 | | 2020 | 4.0359518817572 | | 2019 | 4.0856623183665 | | 2018 | 4.1244902772485 | | 2017 | 4.1668935930239 | | 2016 | 4.2232158295323 | | 2015 | 4.2706079819354 | | 2014 | 4.3223026938264 | | 2013 | 4.3780044060711 | | 2012 | 4.4328727022915 | | 2011 | 4.5038091103319 | | 2010 | 4.5748710380941 | | 2009 | 4.6466833961738 | | 2008 | 4.710377767042 | | 2007 | 4.7757010163851 | | 2006 | 4.837661029422 | | 2005 | 4.8988456177999 | | 2004 | 4.966329655824 | | 2003 | 5.0322420716408 | | 2002 | 5.0959782739561 | | 2001 | 5.1518448888579 | | 2000 | 5.2134978944652 | | 1999 | 5.2829688980979 | | 1998 | 5.3536548301331 | | 1997 | 5.4314501569447 | | 1996 | 5.4855705270645 | | 1995 | 5.5653478739926 | | 1994 | 5.6470110057476 | | 1993 | 5.7106219545466 | | 1992 | 5.7872111048228 | | 1991 | 5.872831026227 | | 1990 | 5.9657573968524 | | 1989 | 6.0494779851011 | | 1988 | 6.1226697870346 | | 1987 | 6.2157519601626 | | 1986 | 6.2992086736474 | | 1985 | 6.3828943983097 | | 1984 | 6.4510851585301 | | 1983 | 6.4996495473562 | | 1982 | 6.5531521496916 | | 1981 | 6.5914782034881 | | 1980 | 6.6209075382861 | | 1979 | 6.6368459771807 | | 1978 | 6.6387173331305 | | 1977 | 6.644468221614 | | 1976 | 6.6712857798616 | | 1975 | 6.6963308660255 | | 1974 | 6.7220792296941 | | 1973 | 6.7416412108807 | | 1972 | 6.7637618180228 | | 1971 | 6.7655317363827 | | 1970 | 6.7598749855516 | | 1969 | 6.7467687846493 | | 1968 | 6.7410973170917 | | 1967 | 6.7320205384986 | | 1966 | 6.7162053362213 | | 1965 | 6.708111925387 | | 1964 | 6.6997436125275 | | 1963 | 6.6876358964886 | | 1962 | 6.6824796669488 | | 1961 | 6.6624599724984 | | 1960 | 6.6407354414286 |
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