| Year | Value | | 2019 | 16.546880510909 | | 2018 | 19.32266369536 | | 2017 | 18.243022815334 | | 2016 | 21.30820021043 | | 2015 | 24.540336230391 | | 2014 | 24.843851836611 | | 2013 | 24.134903235796 | | 2012 | 24.790210237761 | | 2011 | 25.643742201293 | | 2010 | 26.244018461364 | | 2009 | 26.827406061286 | | 2008 | 29.48840531425 | | 2007 | 25.995290337891 | | 2006 | 30.257587654351 | | 2005 | 26.378700795322 | | 2004 | 22.025056455906 | | 2003 | 21.30154983209 | | 2002 | 20.455498312311 | | 2001 | 22.12128594075 | | 1981 | 34.844169000655 | | 1980 | 35.650808499265 | | 1979 | 29.18213283947 | | 1978 | 28.749882026649 | | 1977 | 32.621336023789 | | 1976 | 29.588703233089 |
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