| Year | Value | | 2021 | 1.827933856217 | | 2020 | 1.3588569273893 | | 2019 | 1.1709683824213 | | 2018 | 1.0067458557056 | | 2017 | 0.94522679993952 | | 2016 | 0.95677214095885 | | 2015 | 0.99457582272364 | | 2014 | 1.2980128779002 | | 2013 | 1.076949767624 | | 2012 | 1.1754161200999 | | 2011 | 1.8213454535463 | | 2010 | 1.9458356941881 | | 2009 | 2.0874125527838 | | 2008 | 2.335545701939 | | 2007 | 2.5662673519857 | | 2006 | 1.8962339963425 | | 2005 | 1.9920656985609 | | 2004 | 2.4312537873912 | | 1977 | 2.0114747930775 | | 1976 | 2.0460869565217 | | 1975 | 1.7220657276995 | | 1974 | 1.610824742268 | | 1973 | 1.8689102564103 | | 1970 | 1.6296060991106 |
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