| Year | Value | | 2021 | 3.1425652048234 | | 2020 | 3.1860027786024 | | 2019 | 3.2245189450334 | | 2018 | 3.2799937531451 | | 2017 | 3.3258819325524 | | 2016 | 3.3713578131179 | | 2015 | 3.4710380972074 | | 2014 | 3.50017425229 | | 2013 | 3.5135181565557 | | 2012 | 3.5128150603717 | | 2011 | 3.5080213568436 | | 2010 | 3.504198074315 | | 2009 | 3.4927800347551 | | 2008 | 3.508533048583 | | 2007 | 3.5321491512517 | | 2006 | 3.5475092183508 | | 2005 | 3.5765957058439 | | 2004 | 3.6166839863267 | | 2003 | 3.6706902981986 | | 2002 | 3.7273688442037 | | 2001 | 3.8077833507959 | | 2000 | 3.8785887451054 | | 1999 | 3.9627025473305 | | 1998 | 4.0616400067826 | | 1997 | 4.1651632994564 | | 1996 | 4.2843718794796 | | 1995 | 4.442823499229 | | 1994 | 4.6079872073115 | | 1993 | 4.7476001233028 | | 1992 | 4.8938891936685 | | 1991 | 5.0062599569478 | | 1990 | 5.1477257387344 | | 1989 | 5.2890319766094 | | 1988 | 5.4504767842884 | | 1987 | 5.6002789812738 | | 1986 | 5.7374962246606 | | 1985 | 5.8724177552208 | | 1984 | 5.9869725582358 | | 1983 | 6.096442952714 | | 1982 | 6.1850352249935 | | 1981 | 6.266879970795 | | 1980 | 6.353993301829 | | 1979 | 6.4373651995994 | | 1978 | 6.474031012532 | | 1977 | 6.5235191100248 | | 1976 | 6.5808261700461 | | 1975 | 6.6363087342723 | | 1974 | 6.6808898126207 | | 1973 | 6.7270905560255 | | 1972 | 6.7865777712764 | | 1971 | 6.8367975669809 | | 1970 | 6.8856461884557 | | 1969 | 6.9237357471817 | | 1968 | 6.9576972905387 | | 1967 | 6.9875373256878 | | 1966 | 7.0129606016744 | | 1965 | 7.0335603663955 | | 1964 | 7.0619688483084 | | 1963 | 7.0715416831474 | | 1962 | 7.0761122246693 | | 1961 | 6.9821150297303 | | 1960 | 6.9343324173624 |
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