| Year | Value | | 2021 | 1.6645220374827 | | 2020 | 1.6712894069452 | | 2019 | 1.6878697542451 | | 2018 | 1.7118362295442 | | 2017 | 1.7402008684079 | | 2016 | 1.7842368577611 | | 2015 | 1.8270019422992 | | 2014 | 1.8334523639625 | | 2013 | 1.8595725330384 | | 2012 | 1.9057858409368 | | 2011 | 1.926137334484 | | 2010 | 1.9420483958414 | | 2009 | 2.0113088783988 | | 2008 | 2.0365682701451 | | 2007 | 2.0427558024324 | | 2006 | 2.1110350884708 | | 2005 | 2.1331857861171 | | 2004 | 2.1651458597328 | | 2003 | 2.1843145479922 | | 2002 | 2.2153265998083 | | 2001 | 2.2523370590295 | | 2000 | 2.2852508846179 | | 1999 | 2.3923483271024 | | 1998 | 2.471147741446 | | 1997 | 2.5216231744692 | | 1996 | 2.5452738009996 | | 1995 | 2.6551621669922 | | 1994 | 2.7056814100185 | | 1993 | 2.7584636218514 | | 1992 | 2.7955580006876 | | 1991 | 2.7935918688784 | | 1990 | 2.8045363150874 | | 1989 | 2.8997547546398 | | 1988 | 2.9671809396558 | | 1987 | 3.0483634006124 | | 1986 | 3.1334725010531 | | 1985 | 3.2092095297862 | | 1984 | 3.2682195620491 | | 1983 | 3.3595485204873 | | 1982 | 3.4334940650738 | | 1981 | 3.4968099109729 | | 1980 | 3.563321653483 | | 1979 | 3.6447194978528 | | 1978 | 3.7261970232881 | | 1977 | 3.8264887117271 | | 1976 | 3.9236704301905 | | 1975 | 4.0952284981954 | | 1974 | 4.285027659758 | | 1973 | 4.4866094968405 | | 1972 | 4.6771374900512 | | 1971 | 4.8185360773378 | | 1970 | 4.933826079801 | | 1969 | 5.0489052674935 | | 1968 | 5.1854980415811 | | 1967 | 5.2859393001431 | | 1966 | 5.3927076408939 | | 1965 | 5.4609764913879 | | 1964 | 5.5358075501985 | | 1963 | 5.5518305727871 | | 1962 | 5.5593662979293 | | 1961 | 5.627889102753 | | 1960 | 5.7279132297078 |
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