| Year | Value | | 2022 | 11.665567478802 | | 2021 | 7.2409776558844 | | 2020 | 1.8903591682421 | | 2019 | 2.7728644286332 | | 2018 | 3.2380155894772 | | 2017 | 3.3082807676822 | | 2016 | 2.8149579136194 | | 2015 | 3.0620316958056 | | 2014 | 4.4022530924343 | | 2013 | 5.8846070705682 | | 2012 | 7.5369026109192 | | 2011 | 8.4598723340858 | | 2010 | 6.948876587431 | | 2009 | 8.0272970310958 | | 2008 | 12.702188392008 | | 2007 | 7.0809984717269 | | 2006 | 11.555218791438 | | 2005 | 8.6102252854644 | | 2004 | 6.9457036365468 | | 2003 | 9.1899026563274 | | 2002 | 8.0327966252749 | | 2001 | 6.5590376701486 | | 2000 | 8.6014851485149 | | 1999 | 7.7492961920285 | | 1998 | 6.6613986566575 | | 1997 | 8.7199312714775 | | 1996 | 10.082858065211 | | 1995 | 10.512546081579 | | 1994 | 10.542920993822 | | 1993 | 14.330803336589 | | 1992 | 16.167612396334 | | 1991 | 11.765016083953 | | 1990 | 11.396346119956 | | 1989 | 11.575014747329 | | 1988 | 8.3516795682122 | | 1987 | 9.8019338739867 | | 1986 | 10.001715560602 | | 1985 | 8.0945757997594 | | 1984 | 8.5774690425191 | | 1983 | 10.477143810524 | | 1982 | 11.137206427469 | | 1981 | 16.428026772571 | | 1980 | 13.632425147089 | | 1979 | 11.736230636789 | | 1978 | 9.0439882701036 | | 1977 | 13.168724280108 | | 1976 | 11.732423613642 | | 1975 | 11.974755024095 |
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