| Year | Value | | 2021 | 7.7159786917373 | | 2020 | 7.6534761446601 | | 2019 | 7.5136155580541 | | 2018 | 7.4925325818581 | | 2017 | 7.4591649120349 | | 2016 | 7.4942944757906 | | 2015 | 7.5224112132668 | | 2014 | 7.4333940294867 | | 2013 | 7.4328011452849 | | 2012 | 7.4135660069892 | | 2011 | 7.3455557482097 | | 2010 | 7.2818973086113 | | 2009 | 7.0826156968637 | | 2008 | 7.1020295039452 | | 2007 | 7.1327759363537 | | 2006 | 7.0141650901676 | | 2005 | 6.8973719805529 | | 2004 | 6.7698732791843 | | 2003 | 6.6722145696578 | | 2002 | 6.5147080499463 | | 2001 | 6.4802319873675 | | 2000 | 6.4366491433181 | | 1999 | 6.3965684027292 | | 1998 | 6.3477620783574 | | 1997 | 6.2752448545372 | | 1996 | 6.2072309215846 | | 1995 | 6.1726752271421 | | 1994 | 6.1790947068492 | | 1993 | 6.2107936936106 | | 1992 | 6.2536930864168 | | 1991 | 6.3037721120926 | | 1990 | 6.3246045079063 | | 1989 | 6.343095180774 | | 1988 | 6.3612819278977 | | 1987 | 6.3875401456014 | | 1986 | 6.3984416431681 | | 1985 | 6.4293383207714 | | 1984 | 6.335365651568 | | 1983 | 6.2753333716645 | | 1982 | 6.1927358558439 | | 1981 | 6.1288721136272 | | 1980 | 6.0722382217813 | | 1979 | 5.9194467759154 | | 1978 | 5.7849333623374 | | 1977 | 5.6359063089819 | | 1976 | 5.4780604761688 | | 1975 | 5.3107635666979 | | 1974 | 5.2249582377798 | | 1973 | 5.1564744345658 | | 1972 | 5.072410767259 | | 1971 | 4.9662364756074 | | 1970 | 4.9015653209553 | | 1969 | 4.9092879935062 | | 1968 | 4.8795930719556 | | 1967 | 4.7698812980388 | | 1966 | 4.7041140221213 | | 1965 | 4.6309230158162 | | 1964 | 4.5853841596128 | | 1963 | 4.5224568342463 | | 1962 | 4.4597288036554 | | 1961 | 4.4040259139011 |
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