| Year | Value | | 2020 | 2.2874084964887 | | 2019 | 2.1874065669356 | | 2018 | 2.2123820548059 | | 2017 | 2.1597780439426 | | 2016 | 2.2084337075707 | | 2015 | 2.3391748873037 | | 2014 | 2.332262491281 | | 2013 | 2.3582139647716 | | 2012 | 2.9665953537254 | | 2011 | 3.044429541256 | | 2010 | 3.1945675272585 | | 2009 | 3.2362092411307 | | 2008 | 3.3171347539382 | | 2007 | 3.2150508191283 | | 2006 | 3.6758483984954 | | 2005 | 3.2523191401955 | | 2004 | 3.2537764079948 | | 2003 | 3.6307679684582 | | 2002 | 3.7521274587577 | | 2001 | 3.8559103971436 | | 2000 | 3.8591646521738 | | 1999 | 3.8667077624461 | | 1998 | 4.1928704170079 | | 1997 | 4.2628771086918 | | 1996 | 4.3171786867891 | | 1995 | 4.3218776088466 | | 1994 | 3.2540406717025 | | 1993 | 3.4580439297989 | | 1992 | 3.5931515541754 | | 1991 | 3.9657987680349 |
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