| Year | Value | | 2020 | 2.879 | | 2019 | 2.759 | | 2018 | 2.664 | | 2017 | 2.548 | | 2016 | 2.468 | | 2015 | 2.297 | | 2014 | 2.081 | | 2013 | 1.83 | | 2012 | 1.558 | | 2011 | 1.499 | | 2010 | 1.431 | | 2009 | 1.503 | | 2008 | 1.664 | | 2007 | 1.476 | | 2006 | 1.439 | | 2005 | 1.628 | | 2004 | 1.954 | | 2003 | 2.021 | | 2002 | 1.917 | | 2001 | 1.896 | | 2000 | 1.724 | | 1984 | 0.953 | | 1980 | 1.068 | | 1970 | 0.909 |
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