| Year | Value | | 2022 | 12.323602479589 | | 2021 | 11.435610497414 | | 2020 | 10.338464556922 | | 2019 | 9.9593441765632 | | 2018 | 9.9892420735674 | | 2017 | 11.583483164262 | | 2016 | 10.918266905454 | | 2015 | 11.237494063668 | | 2014 | 10.935432022989 | | 2013 | 10.483509334626 | | 2012 | 9.763916029528 | | 2011 | 9.5389309882013 | | 2010 | 8.5632257607851 | | 2009 | 7.9035182202616 | | 2008 | 10.276190936608 | | 2007 | 11.289913518182 | | 2006 | 11.310475666951 | | 2005 | 10.681731135358 | | 2004 | 9.5374729785787 | | 2003 | 9.3906991031754 | | 2002 | 9.8680122022767 | | 2001 | 11.85770390466 | | 2000 | 12.970800362943 | | 1999 | 12.259979313516 | | 1998 | 12.329829144685 | | 1997 | 11.828196545835 | | 1996 | 11.373914577285 | | 1995 | 10.980989035111 | | 1994 | 10.623506635438 | | 1993 | 10.304934316377 | | 1992 | 9.9841618372821 | | 1991 | 10.320342428682 | | 1990 | 10.465284755827 | | 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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