| Year | Value | | 2021 | 10.131769766435 | | 2020 | 10.073496881973 | | 2019 | 8.5869369371929 | | 2018 | 8.5783376868676 | | 2017 | 8.5228283980463 | | 2016 | 8.3829949253324 | | 2015 | 8.3357123276569 | | 2014 | 8.1430910310745 | | 2013 | 8.1134955243832 | | 2012 | 8.001993262025 | | 2011 | 7.9761919696214 | | 2010 | 7.8963573270974 | | 2009 | 7.8208465560414 | | 2008 | 8.0110649207478 | | 2007 | 7.9112318031994 | | 2006 | 7.991567607617 | | 2005 | 8.1817370645256 | | 2004 | 8.0915550943857 | | 2003 | 8.2719351497803 | | 2002 | 8.3620354090466 | | 2001 | 8.3623312989584 | | 2000 | 8.3626092420435 | | 1999 | 8.462455973635 | | 1998 | 8.4620392918744 | | 1997 | 8.5517368954245 | | 1996 | 8.6412027566094 | | 1995 | 8.6412344068788 | | 1994 | 8.6311258788208 | | 1993 | 8.6309464672942 | | 1992 | 8.340567964653 | | 1991 | 8.4402113284502 | | 1990 | 8.4699385201463 | | 1989 | 8.6505226856981 | | 1988 | 8.7419894682217 | | 1987 | 8.4622242782042 | | 1986 | 8.5529469445213 | | 1985 | 8.5334448796297 | | 1984 | 8.5331770961103 | | 1983 | 8.443391865311 | | 1982 | 8.3630594680192 | | 1981 | 8.4438298715837 | | 1980 | 8.6341564248269 | | 1979 | 8.3539790390727 | | 1978 | 8.5439352659559 | | 1977 | 8.4537176398733 | | 1976 | 8.6442228901714 | | 1975 | 8.6544754024111 | | 1974 | 8.9355701724631 | | 1973 | 9.1171548755903 | | 1972 | 9.2081104253075 | | 1971 | 9.1084962666282 | | 1970 | 9.2923548454054 | | 1969 | 9.2932026552826 | | 1968 | 9.575183562881 | | 1967 | 9.2136945291251 | | 1966 | 9.3143935862346 | | 1965 | 9.2344694155088 | | 1964 | 9.2261597242607 | | 1963 | 9.4360508342699 | | 1962 | 9.3362412803986 | | 1961 | 9.1552519780949 | | 1960 | 9.3468058775111 |
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