| Year | Value | | 2021 | 5.5153136480611 | | 2020 | 5.051636559307 | | 2019 | 6.5249850544656 | | 2018 | 6.389419705049 | | 2017 | 6.2753948939269 | | 2016 | 5.1411010779868 | | 2015 | 5.8243637706327 | | 2014 | 4.6404304386503 | | 2013 | 4.7102606361534 | | 2012 | 5.7673336021354 | | 2011 | 5.5783447356277 | | 2010 | 4.9718622175197 | | 2009 | 4.8425372538869 | | 2008 | 6.0331931716479 | | 2007 | 4.7761709652145 | | 2006 | 4.4720143903363 | | 2005 | 4.7927921405214 | | 2004 | 4.265176181261 |
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