| Year | Value | | 2021 | 2.1487133600567 | | 2020 | 2.1857463456692 | | 2019 | 2.2571203746981 | | 2018 | 2.3009937477686 | | 2017 | 2.3768808799567 | | 2016 | 2.399075361999 | | 2015 | 2.3943275935609 | | 2014 | 2.4311502260204 | | 2013 | 2.4430552062997 | | 2012 | 2.4818222365621 | | 2011 | 2.4625311086119 | | 2010 | 2.4851977167293 | | 2009 | 2.5144931202843 | | 2008 | 2.5276118790422 | | 2007 | 2.5310540281826 | | 2006 | 2.5388646316821 | | 2005 | 2.5594001352415 | | 2004 | 2.5842496010931 | | 2003 | 2.5989914432896 | | 2002 | 2.633473270621 | | 2001 | 2.6674557918211 | | 2000 | 2.7107191930538 | | 1999 | 2.7072652812407 | | 1998 | 2.7441575160497 | | 1997 | 2.7899056828674 | | 1996 | 2.8450894984613 | | 1995 | 2.9075003114268 | | 1994 | 2.9728842853078 | | 1993 | 3.0428882132151 | | 1992 | 3.1286042498635 | | 1991 | 3.2411900643149 | | 1990 | 3.4782946032485 | | 1989 | 3.5402490898711 | | 1988 | 3.6084745054175 | | 1987 | 3.7412325857987 | | 1986 | 3.7855808458133 | | 1985 | 3.8047626121089 | | 1984 | 3.8617665720974 | | 1983 | 3.8979932748149 | | 1982 | 4.0693363213651 | | 1981 | 4.0504618645859 | | 1980 | 4.0793652184939 | | 1979 | 4.1186359681336 | | 1978 | 4.1548889936025 | | 1977 | 4.2460611136366 | | 1976 | 4.4189423180427 | | 1975 | 4.5584698814209 | | 1974 | 4.7930161916164 | | 1973 | 5.0111243906843 | | 1972 | 5.1722614220666 | | 1971 | 5.3369107585158 | | 1970 | 5.5380262873008 | | 1969 | 5.5937700933634 | | 1968 | 5.7248121294064 | | 1967 | 5.5452727694343 | | 1966 | 5.7268593993347 | | 1965 | 5.8445325564541 | | 1964 | 5.886937807981 | | 1963 | 6.1496547208077 | | 1962 | 5.7290005290454 | | 1961 | 5.0481481526717 | | 1960 | 5.223102929962 |
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