| Year | Value | | 2021 | 7.6148092023515 | | 2020 | 7.5519143643705 | | 2019 | 7.4111759569637 | | 2018 | 7.3899646681402 | | 2017 | 7.3563621593326 | | 2016 | 7.3877098058334 | | 2015 | 7.4147002733635 | | 2014 | 7.3212681382779 | | 2013 | 7.3172407725522 | | 2012 | 7.2997981483237 | | 2011 | 7.2221652484087 | | 2010 | 7.1549448639003 | | 2009 | 6.9514283481788 | | 2008 | 6.968028885941 | | 2007 | 6.9959442858038 | | 2006 | 6.8731600543788 | | 2005 | 6.7533220459231 | | 2004 | 6.6230476339022 | | 2003 | 6.5221084079489 | | 2002 | 6.360938074983 | | 2001 | 6.329638074059 | | 2000 | 6.297835909592 | | 1999 | 6.2524797083726 | | 1998 | 6.2051710757736 | | 1997 | 6.1304091913635 | | 1996 | 6.069328063146 | | 1995 | 6.0245706823831 | | 1994 | 6.030931262446 | | 1993 | 6.0655926162685 | | 1992 | 6.1105236897677 | | 1991 | 6.1677954922728 | | 1990 | 6.190467143099 | | 1989 | 6.2082747153361 | | 1988 | 6.2272807564214 | | 1987 | 6.2569175672065 | | 1986 | 6.2718508200134 | | 1985 | 6.3071087555822 | | 1984 | 6.2153884553451 | | 1983 | 6.1554204138458 | | 1982 | 6.0746471587362 | | 1981 | 6.0128789016931 | | 1980 | 5.9565818856097 | | 1979 | 5.8017026304317 | | 1978 | 5.6634135187278 | | 1977 | 5.5137766080195 | | 1976 | 5.3552139126863 | | 1975 | 5.1947118421256 | | 1974 | 5.1200362277126 | | 1973 | 5.0436894148848 | | 1972 | 4.9668060562249 | | 1971 | 4.8758932819421 | | 1970 | 4.8080471795678 | | 1969 | 4.833520808545 | | 1968 | 4.803534135064 | | 1967 | 4.7055509243919 | | 1966 | 4.6455775774298 | | 1965 | 4.567100748002 | | 1964 | 4.5153511710247 | | 1963 | 4.459489709055 | | 1962 | 4.4007699855663 | | 1961 | 4.3469143211539 |
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