| Year | Value | | 2021 | 2.4917869423947 | | 2020 | 2.4677437124086 | | 2019 | 2.6326671709505 | | 2018 | 2.8312492420569 | | 2017 | 3.2382920536428 | | 2016 | 3.3508876008721 | | 2015 | 3.9036080582406 | | 2014 | 4.2061074945605 | | 2013 | 5.2981605408391 | | 2012 | 4.7398598156196 | | 2011 | 5.7126889741153 | | 2010 | 6.0017923304429 | | 2009 | 5.5725286372285 | | 2008 | 5.24529396706 | | 2007 | 4.5898695026968 | | 2006 | 4.9682601889394 | | 2005 | 4.9563921902127 | | 2004 | 5.7697252855827 | | 2003 | 5.9716439359057 | | 2002 | 6.3493606094813 | | 2001 | 6.1835333796416 | | 2000 | 6.2406453084905 | | 1999 | 6.1953750071093 | | 1998 | 5.6579934088074 | | 1997 | 8.2913854250086 | | 1996 | 8.1725958652114 | | 1995 | 8.1604255469053 | | 1994 | 7.3062355853664 | | 1993 | 7.3203543417011 | | 1992 | 8.8529137067331 | | 1991 | 10.836595716683 | | 1990 | 13.692267677331 | | 1989 | 13.025091626727 | | 1988 | 16.224521151286 | | 1987 | 19.571323888617 | | 1986 | 17.228986357635 | | 1985 | 16.390686727746 | | 1984 | 17.591575091575 | | 1983 | 17.852535476828 | | 1982 | 15.643188027082 | | 1981 | 16.336905743336 | | 1980 | 17.407725321888 | | 1979 | 19.10841356216 | | 1978 | 21.169079486479 | | 1977 | 18.060836501901 | | 1976 | 16.435738174869 | | 1975 | 14.501076812635 | | 1974 | 13.879781420765 | | 1973 | 14.162348877375 | | 1972 | 12.394957983193 |
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