| Year | Value | | 2021 | 11.409783713166 | | 2020 | 11.351720133546 | | 2019 | 11.293656553926 | | 2018 | 11.235592974307 | | 2017 | 11.177529394687 | | 2016 | 11.066192480766 | | 2015 | 10.954710407897 | | 2014 | 10.855160400639 | | 2013 | 10.755610393381 | | 2012 | 10.656060386123 | | 2011 | 10.556510378865 | | 2010 | 10.456960371607 | | 2009 | 10.326288285673 | | 2008 | 10.195616199739 | | 2007 | 10.064944113805 | | 2006 | 9.9342720278705 | | 2005 | 9.8035999419364 | | 2004 | 9.6729278560023 | | 2003 | 9.5422557700682 | | 2002 | 9.4115836841341 | | 2001 | 9.2809115982 | | 2000 | 9.1502395122659 | | 1999 | 8.9053273334301 | | 1998 | 8.6604151545943 | | 1997 | 8.4155029757585 | | 1996 | 8.1705907969226 | | 1995 | 7.9256786180868 | | 1994 | 7.680766439251 | | 1993 | 7.4358542604152 | | 1992 | 7.1909420815793 | | 1991 | 6.9460299027435 | | 1990 | 6.7011177239077 |
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