| Year | Value | | 2022 | 12.366497604426 | | 2021 | 11.587948197435 | | 2020 | 10.608673052167 | | 2019 | 10.196893828393 | | 2018 | 10.251218047913 | | 2017 | 11.671296790159 | | 2016 | 11.053017857497 | | 2015 | 11.335876001146 | | 2014 | 11.009962065013 | | 2013 | 10.58083928649 | | 2012 | 9.926176946209 | | 2011 | 9.7323102092209 | | 2010 | 8.8338368964249 | | 2009 | 8.2792533291205 | | 2008 | 10.456676819587 | | 2007 | 11.464469406532 | | 2006 | 11.484453251935 | | 2005 | 10.905229103383 | | 2004 | 9.8625286110024 | | 2003 | 9.7183980847754 | | 2002 | 10.167654398687 | | 2001 | 12.056102095033 | | 2000 | 13.131902833925 | | 1999 | 12.468295424818 | | 1998 | 12.529490284715 | | 1997 | 12.062090937105 | | 1996 | 11.587749220232 | | 1995 | 11.177875813983 | | 1994 | 10.832697565218 | | 1993 | 10.590727107082 | | 1992 | 10.353393761116 | | 1991 | 10.668454963753 | | 1990 | 10.776643240714 | | 1989 | 10.782440380177 | | 1988 | 10.595943272889 | | 1987 | 10.948845725679 | | 1986 | 10.163045887322 | | 1985 | 10.384240163412 | | 1984 | 10.159715653779 | | 1983 | 10.34826823495 | | 1982 | 11.978626641813 | | 1981 | 12.550509956062 | | 1980 | 12.13835265164 | | 1979 | 12.003807663513 | | 1978 | 11.542359050161 | | 1977 | 11.618646322988 | | 1976 | 10.748836881583 | | 1975 | 11.185800496646 | | 1974 | 11.837620361328 | | 1973 | 11.4952124913 | | 1972 | 11.819937300154 |
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