| Year | Value | | 2021 | 9.094042440814 | | 2020 | 8.3101052419225 | | 2019 | 9.7519618082787 | | 2018 | 10.230143021132 | | 2017 | 9.8772930803788 | | 2016 | 10.336348703328 | | 2015 | 10.75348777721 | | 2014 | 10.835523753291 | | 2013 | 11.285301463399 | | 2012 | 11.380589944792 | | 2011 | 11.158127850744 | | 2010 | 10.537485061296 | | 2009 | 11.057786518801 | | 2008 | 13.310617826992 | | 2004 | 12.330767895235 | | 2003 | 12.385532064309 | | 2002 | 11.82695958917 | | 2001 | 11.578184583575 | | 1999 | 16.315835609343 | | 1998 | 15.027588180425 | | 1997 | 16.011848414471 | | 1996 | 14.234801940785 | | 1995 | 14.964772269991 | | 1994 | 15.94841919451 | | 1993 | 14.356420331631 | | 1992 | 17.122952696294 | | 1991 | 17.189696909477 | | 1990 | 19.137083761935 | | 1989 | 15.957201825287 | | 1988 | 15.083938051354 | | 1987 | 15.083694595844 | | 1986 | 14.620769821124 | | 1985 | 18.750897374178 | | 1984 | 17.48438914339 | | 1983 | 18.822865330757 | | 1982 | 20.056143787123 | | 1981 | 21.946434190312 | | 1980 | 21.779838356544 | | 1979 | 20.283899654649 | | 1978 | 17.893255957091 | | 1977 | 17.721598889047 | | 1976 | 17.890047650759 | | 1975 | 16.555269922879 | | 1974 | 15.969368696302 | | 1973 | 13.815263422868 | | 1972 | 12.445223488168 |
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