| Year | Value | | 2021 | 3.8438960350554 | | 2020 | 3.8927192622348 | | 2019 | 3.9421369860248 | | 2018 | 3.9798771591646 | | 2017 | 4.0178996118234 | | 2016 | 4.0741844296091 | | 2015 | 4.1211135945593 | | 2014 | 4.1657830222107 | | 2013 | 4.2153258249332 | | 2012 | 4.2630808256555 | | 2011 | 4.3257100424345 | | 2010 | 4.3919400328778 | | 2009 | 4.4573883337497 | | 2008 | 4.5173601551448 | | 2007 | 4.5802882636251 | | 2006 | 4.6429561960593 | | 2005 | 4.7019053572553 | | 2004 | 4.7648996485583 | | 2003 | 4.8288928352608 | | 2002 | 4.8918917299129 | | 2001 | 4.9470435961952 | | 2000 | 5.0039985832601 | | 1999 | 5.0679509574144 | | 1998 | 5.1327361204921 | | 1997 | 5.20545739749 | | 1996 | 5.2636925660088 | | 1995 | 5.3486269915141 | | 1994 | 5.4292516820935 | | 1993 | 5.4996149393518 | | 1992 | 5.5808604734766 | | 1991 | 5.6703642675782 | | 1990 | 5.7665181854556 | | 1989 | 5.8551312043544 | | 1988 | 5.9341211737345 | | 1987 | 6.0314104353553 | | 1986 | 6.1186951443034 | | 1985 | 6.2044955270559 | | 1984 | 6.2761510025153 | | 1983 | 6.3297567989351 | | 1982 | 6.3916131097557 | | 1981 | 6.4429606271623 | | 1980 | 6.4808439524693 | | 1979 | 6.5046498254607 | | 1978 | 6.5129681610959 | | 1977 | 6.5254110333447 | | 1976 | 6.5574201024427 | | 1975 | 6.5857332035643 | | 1974 | 6.6160343376587 | | 1973 | 6.6452102016478 | | 1972 | 6.6746275018303 | | 1971 | 6.6835007652085 | | 1970 | 6.685466354731 | | 1969 | 6.6817893841205 | | 1968 | 6.6826392879024 | | 1967 | 6.6829987856452 | | 1966 | 6.6707280086994 | | 1965 | 6.6705448334442 | | 1964 | 6.6699846106935 | | 1963 | 6.6644286400181 | | 1962 | 6.6614956363069 | | 1961 | 6.6447179939316 | | 1960 | 6.6297526475113 |
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