| Year | Value | | 2020 | 3.1320669739 | | 2019 | 3.0513024336794 | | 2018 | 3.0460909741001 | | 2017 | 3.0275223265122 | | 2016 | 3.0302988926096 | | 2015 | 3.083888400715 | | 2014 | 3.088065370044 | | 2013 | 3.0906199098908 | | 2012 | 3.0929401097172 | | 2011 | 3.0973329023205 | | 2010 | 3.5953471561795 | | 2009 | 3.6086819039811 | | 2008 | 3.6289535763635 | | 2007 | 3.6498963569048 | | 2006 | 3.6696791457147 | | 2005 | 3.6852097830841 | | 2004 | 3.6801452322809 | | 2003 | 3.7153071263003 | | 2002 | 3.8779686227797 | | 2001 | 3.9739249544152 | | 2000 | 1.8979866549246 | | 1999 | 2.1940915795664 | | 1998 | 1.9319162974973 | | 1997 | 1.9094536240919 | | 1996 | 1.9825278863221 | | 1995 | 2.254473243598 | | 1994 | 1.3838555578407 | | 1993 | 2.4594411422586 | | 1992 | 2.1887524304272 |
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