| Year | Value | | 2022 | 2.6307402802317 | | 2021 | 2.8719860656416 | | 2020 | 3.0192758324751 | | 2019 | 3.2855154864149 | | 2018 | 3.2673590070103 | | 2017 | 3.0823178270066 | | 2016 | 2.9489574371023 | | 2015 | 2.9853813205902 | | 2014 | 3.4852508889476 | | 2013 | 3.4756138718645 | | 2012 | 3.4843339236276 | | 2011 | 3.2856508160232 | | 2010 | 3.4236909729444 | | 2009 | 3.2652239932295 | | 2008 | 3.4593710036274 | | 2007 | 3.511986100111 | | 2006 | 3.6452552043777 | | 2005 | 3.8970193925629 | | 2004 | 3.9560341166915 | | 2003 | 4.0914271873561 | | 2002 | 4.0737701678766 | | 2001 | 3.8789339447946 | | 2000 | 4.168722713815 | | 1999 | 5.1898992649061 | | 1998 | 5.4150950442544 | | 1997 | 5.6212164889017 | | 1996 | 6.0371663050656 | | 1995 | 5.8202252828579 | | 1994 | 5.9235482929676 | | 1993 | 6.4273126231038 | | 1992 | 6.6981875492514 | | 1991 | 6.5709527881543 | | 1990 | 6.5223202352312 | | 1989 | 6.5327252557624 | | 1988 | 6.785810731275 | | 1987 | 6.9718800836626 | | 1986 | 6.9916560105243 | | 1985 | 6.9221335981919 | | 1984 | 6.659491457016 | | 1983 | 6.6437980591438 | | 1982 | 6.3757170866313 | | 1981 | 5.7108705757354 | | 1980 | 5.5232444669972 | | 1979 | 5.4532742950361 | | 1978 | 5.5032996632997 | | 1977 | 5.5474116781158 | | 1976 | 5.8894719931456 | | 1975 | 6.321520245094 | | 1974 | 6.0909963136898 | | 1973 | 6.2106427270362 | | 1972 | 6.8659555114529 | | 1971 | 6.7355332376853 | | 1970 | 6.1583876221498 | | 1969 | 5.5120418848167 | | 1968 | 5.1039059949951 | | 1967 | 5.4046037019459 | | 1966 | 6.7305219850861 | | 1965 | 5.9046678888732 | | 1964 | 3.8658901306469 | | 1963 | 3.6484129752354 | | 1962 | 3.6558727725618 | | 1961 | 4.0978906409994 | | 1960 | 4.3126626192541 |
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