| Year | Value | | 2021 | 1.8243928687144 | | 2020 | 1.8670372637451 | | 2019 | 1.9503039322248 | | 2018 | 2.0013729736424 | | 2017 | 2.0907247203428 | | 2016 | 2.1106751627995 | | 2015 | 2.1005420383441 | | 2014 | 2.1401816258119 | | 2013 | 2.148370997924 | | 2012 | 2.1910006155353 | | 2011 | 2.1604216917938 | | 2010 | 2.1806355769365 | | 2009 | 2.2070246580598 | | 2008 | 2.215361710287 | | 2007 | 2.2137345078227 | | 2006 | 2.2207132940038 | | 2005 | 2.2376757362248 | | 2004 | 2.2572434479451 | | 2003 | 2.265837289079 | | 2002 | 2.2960355671258 | | 2001 | 2.3299331034399 | | 2000 | 2.3741414723535 | | 1999 | 2.3640047987525 | | 1998 | 2.4006564525731 | | 1997 | 2.4427218798418 | | 1996 | 2.496827837759 | | 1995 | 2.5555965229887 | | 1994 | 2.6206867799406 | | 1993 | 2.6896018748668 | | 1992 | 2.7743535127568 | | 1991 | 2.8890190934913 | | 1990 | 3.1460757786293 | | 1989 | 3.2084911760601 | | 1988 | 3.276281798005 | | 1987 | 3.4126854316686 | | 1986 | 3.4493252754819 | | 1985 | 3.4576309783791 | | 1984 | 3.5079668802962 | | 1983 | 3.5363195286763 | | 1982 | 3.7175030511689 | | 1981 | 3.6903332794775 | | 1980 | 3.7186951002116 | | 1979 | 3.7590875408287 | | 1978 | 3.7949211322311 | | 1977 | 3.8930845385036 | | 1976 | 4.0807847047244 | | 1975 | 4.2318394418587 | | 1974 | 4.4895652405076 | | 1973 | 4.7247678428349 | | 1972 | 4.8991626600765 | | 1971 | 5.0801451243584 | | 1970 | 5.3027075957388 | | 1969 | 5.3668149758571 | | 1968 | 5.5163117975832 | | 1967 | 5.3181410835887 | | 1966 | 5.5025374700843 | | 1965 | 5.634715615371 | | 1964 | 5.6826415368512 | | 1963 | 5.9771087848257 | | 1962 | 5.5128353805479 | | 1961 | 4.7677740554373 | | 1960 | 4.9700945988072 |
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