| Year | Value | | 2021 | 2.351047717191 | | 2020 | 2.3777384371118 | | 2019 | 2.4218395271089 | | 2018 | 2.472331669805 | | 2017 | 2.5008527612124 | | 2016 | 2.5545226440232 | | 2015 | 2.5893743875686 | | 2014 | 2.6210760751987 | | 2013 | 2.6809891950239 | | 2012 | 2.7216681987573 | | 2011 | 2.7612703939461 | | 2010 | 2.8054979389591 | | 2009 | 2.8571028728085 | | 2008 | 2.9022987445695 | | 2007 | 2.9491145019708 | | 2006 | 2.992997715286 | | 2005 | 3.0529090296274 | | 2004 | 3.1149556809178 | | 2003 | 3.1754570733875 | | 2002 | 3.2516194641503 | | 2001 | 3.3276216931415 | | 2000 | 3.3814321578682 | | 1999 | 3.439400966447 | | 1998 | 3.5114258646749 | | 1997 | 3.5922991091829 | | 1996 | 3.6741718519972 | | 1995 | 3.7636209240154 | | 1994 | 3.8569099952204 | | 1993 | 3.9493082660352 | | 1992 | 4.0452279944044 | | 1991 | 4.1497236092729 | | 1990 | 4.2446719841295 | | 1989 | 4.3489185657541 | | 1988 | 4.4533630916478 | | 1987 | 4.5589510172685 | | 1986 | 4.6616059363935 | | 1985 | 4.746373306402 | | 1984 | 4.8514459067083 | | 1983 | 4.9270187310112 | | 1982 | 5.004038658549 | | 1981 | 5.0793010646378 | | 1980 | 5.1600782812354 | | 1979 | 5.2191482378568 | | 1978 | 5.2886717754576 | | 1977 | 5.3723265509488 | | 1976 | 5.4690967631168 | | 1975 | 5.5366891584777 | | 1974 | 5.6329401968841 | | 1973 | 5.7104446415335 | | 1972 | 5.7846097387431 | | 1971 | 5.8561778117117 | | 1970 | 5.9145801810136 | | 1969 | 5.9662242431403 | | 1968 | 6.0194432330376 | | 1967 | 6.0696875529372 | | 1966 | 6.1116523382177 | | 1965 | 6.1557687632754 | | 1964 | 6.1857633152319 | | 1963 | 6.18862287406 | | 1962 | 6.187165933063 | | 1961 | 6.1770072919161 | | 1960 | 6.1665617291832 |
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