| Year | Value | | 2021 | 7.3730207707387 | | 2020 | 7.379478333536 | | 2019 | 7.3776112609889 | | 2018 | 7.3663432902219 | | 2017 | 7.3933006022136 | | 2016 | 7.416747080312 | | 2015 | 7.4349886857854 | | 2014 | 7.4534105072498 | | 2013 | 7.4534350567843 | | 2012 | 7.442191370004 | | 2011 | 7.5197678988815 | | 2010 | 7.5457904054037 | | 2009 | 7.529833208008 | | 2008 | 7.5509458076392 | | 2007 | 7.6616642080308 | | 2006 | 7.7333488486391 | | 2005 | 7.8587969697037 | | 2004 | 7.7409592043201 | | 2003 | 7.6808128449056 | | 2002 | 7.7083083234951 | | 2001 | 7.4971823271829 | | 2000 | 7.3012770422328 | | 1999 | 7.8411213048961 | | 1998 | 7.9319545823793 | | 1997 | 8.000693278853 | | 1996 | 8.0939815097816 | | 1995 | 8.0203329064169 | | 1994 | 8.2388237630656 | | 1993 | 8.2388237630656 | | 1992 | 8.2093643217197 | | 1991 | 8.2437336699565 | | 1990 | 8.3787561094585 | | 1989 | 8.391030876686 | | 1988 | 8.4376749921503 | | 1987 | 8.4818641541691 | | 1986 | 8.5334181765244 | | 1985 | 8.5776073385432 | | 1984 | 8.8648368916656 | | 1983 | 8.9262107278029 | | 1982 | 8.9851296104946 | | 1981 | 9.021953912177 | | 1980 | 9.0931475620962 | | 1979 | 9.1103322362146 | | 1978 | 9.152066444788 | | 1977 | 9.137336724115 | | 1976 | 9.1692511189064 | | 1975 | 9.147156537897 | | 1974 | 9.1692511189064 | | 1973 | 9.1275169103331 | | 1972 | 9.1667961654609 | | 1971 | 9.2257150481527 | | 1970 | 9.2134402809252 | | 1969 | 9.1938006533613 | | 1968 | 9.1348817706695 | | 1967 | 9.2919987911809 | | 1966 | 9.3214582325268 | | 1965 | 9.4491158116923 | | 1964 | 9.6798814355684 | | 1963 | 9.773169666497 | | 1962 | 10.242065774586 | | 1961 | 10.401637748543 |
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