| Year | Value | | 2022 | 7.1986161560979 | | 2021 | 4.038430127303 | | 2020 | 2.9519225316999 | | 2019 | 2.7570972756277 | | 2018 | 3.9756101574069 | | 2017 | 3.4653253940512 | | 2016 | 3.371530035121 | | 2015 | 3.0955172202724 | | 2014 | 4.0186160807868 | | 2013 | 4.8246231426954 | | 2012 | 5.0182725423743 | | 2011 | 5.811492090227 | | 2010 | 4.3974138315007 | | 2009 | 4.3737688706727 | | 2008 | 10.446987512822 | | 2007 | 6.1778068349941 | | 2006 | 5.6871377260796 | | 2005 | 4.8217005071285 | | 2004 | 4.4352610841078 | | 2003 | 5.452932243272 | | 2002 | 4.2402666068864 | | 2001 | 3.7508205747202 | | 2000 | 3.8995298028471 | | 1999 | 5.0232107855624 | | 1998 | 6.420732429713 | | 1997 | 7.0182859986991 | | 1996 | 8.5199519592534 | | 1995 | 10.022173355324 | | 1994 | 9.5932302455346 | | 1993 | 9.9736647602921 | | 1992 | 10.527444011959 | | 1991 | 15.050130775937 | | 1990 | 11.906054601038 | | 1989 | 9.0756240853786 | | 1988 | 8.9830033820899 | | 1987 | 9.75300310171 | | 1986 | 10.437957993521 | | 1985 | 9.3533308127415 | | 1984 | 8.4973714128198 | | 1983 | 10.243663352654 | | 1982 | 10.221727137689 | | 1981 | 12.413261372398 | | 1980 | 14.684502776377 | | 1979 | 11.542415572699 | | 1978 | 9.3141836956082 | | 1977 | 12.156931504103 | | 1976 | 10.122084073422 | | 1975 | 12.879181833834 | | 1974 | 19.070473300647 | | 1973 | 12.081939461294 | | 1972 | 6.0749999995151 | | 1971 | 4.1952983726542 | | 1970 | 3.8126649076679 | | 1969 | 2.9751392473965 | | 1968 | 2.0792218581362 | | 1967 | 1.9777415468267 | | 1966 | 4.4798106972311 |
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