| Year | Value | | 2021 | 11.602131185007 | | 2020 | 11.566678093932 | | 2019 | 11.931779225231 | | 2018 | 12.29459490344 | | 2017 | 12.478002513998 | | 2016 | 12.720260541652 | | 2015 | 13.876128442464 | | 2014 | 13.918980687921 | | 2013 | 13.185350245686 | | 2012 | 12.341446691807 | | 2011 | 13.438464175523 | | 2010 | 11.61581533539 | | 2009 | 12.541423837276 | | 2008 | 10.433093360759 | | 2007 | 8.4333219060679 | | 2006 | 7.9762312878528 | | 2005 | 7.9533767569421 | | 2004 | 7.936235858759 | | 2003 | 7.9133813278483 | | 2002 | 7.8905267969375 | | 2001 | 7.8676722660267 | | 2000 | 7.844817735116 | | 1999 | 7.7762541423837 | | 1998 | 7.6962632841961 | | 1997 | 7.6162724260085 | | 1996 | 7.5362815678208 | | 1995 | 7.4620043423609 | | 1994 | 7.3877271169009 | | 1993 | 7.313449891441 | | 1992 | 7.239172665981 | | 1991 | 7.1648954405211 | | 1990 | 7.0906182150611 | | 1989 | 7.1820363387042 | | 1988 | 7.2848817278025 | | 1987 | 7.3762998514456 | | 1986 | 7.4734316078163 | | 1985 | 7.5534224660039 | | 1984 | 7.6448405896469 | | 1983 | 7.7362587132899 | | 1982 | 7.8276768369329 | | 1981 | 7.9248085933036 | | 1980 | 8.0162267169466 | | 1979 | 8.2447720260542 | | 1978 | 8.4790309678894 | | 1977 | 8.7075762769969 | | 1976 | 8.9361215861044 | | 1975 | 9.164666895212 | | 1974 | 9.3932122043195 | | 1973 | 9.621757513427 | | 1972 | 9.8445891898069 | | 1971 | 10.078848131642 | | 1970 | 10.30739344075 | | 1969 | 11.118729288081 | | 1968 | 11.54153810993 | | 1967 | 11.958633299052 | | 1966 | 12.375728488173 | | 1965 | 12.701405553651 | | 1964 | 13.032796251857 | | 1963 | 13.358473317335 | | 1962 | 13.689864015541 | | 1961 | 14.021254713747 |
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