| Year | Value | | 2021 | 17.511109524855 | | 2020 | 11.245824594246 | | 2019 | 22.301948646462 | | 2018 | 27.327376080036 | | 2017 | 20.566495332814 | | 2016 | 19.387267629037 | | 2015 | 22.459795865095 | | 2014 | 33.487352284332 | | 2013 | 38.153263120149 | | 2012 | 39.176760779203 | | 2011 | 39.205578935358 | | 2010 | 40.002337735094 | | 2009 | 27.168915257587 | | 2008 | 44.739947715333 | | 2007 | 39.936433882836 | | 2006 | 42.379145029389 | | 2005 | 38.092396219243 | | 2004 | 31.461165397468 | | 2003 | 29.92938294725 | | 2002 | 27.444563865125 | | 2001 | 27.291044387584 | | 2000 | 31.421437347375 | | 1999 | 18.874931074927 | | 1998 | 12.171133937008 | | 1997 | 16.226306880754 | | 1996 | 15.586110802189 | | 1995 | 9.9399935934879 | | 1994 | 10.644096390393 | | 1993 | 8.5112582229994 | | 1992 | 12.730167714447 | | 1991 | 14.03849752339 | | 1990 | 21.889781655501 | | 1989 | 26.334773559774 | | 1988 | 21.423476507986 | | 1987 | 28.590934560529 | | 1986 | 17.818211806697 | | 1985 | 27.740962636099 | | 1984 | 33.37820415169 | | 1983 | 32.895365975075 | | 1982 | 33.222199213162 | | 1981 | 42.001354352729 | | 1980 | 38.652543426747 | | 1979 | 40.504071340094 | | 1978 | 27.247762552592 | | 1977 | 41.518806350634 | | 1976 | 37.28371591455 | | 1975 | 38.359546859801 | | 1974 | 33.632365859832 |
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