| Year | Value | | 2021 | 10.2385306991 | | 2020 | 9.5207287253425 | | 2019 | 10.531864621218 | | 2018 | 10.28312867926 | | 2017 | 10.027544729967 | | 2016 | 9.2448518734724 | | 2015 | 8.94531037345 | | 2014 | 8.4411333821429 | | 2013 | 8.7855917729325 | | 2012 | 8.3682555430777 | | 2011 | 8.7103236377339 | | 2010 | 8.535319117556 | | 2009 | 8.3441489612085 | | 2008 | 11.391450838192 | | 2007 | 9.8480613245244 | | 2006 | 9.2018816209268 | | 2005 | 8.6901555176454 | | 2004 | 9.3724198682296 | | 2003 | 8.5978002861277 | | 1995 | 6.8458658173511 | | 1994 | 6.4995328552394 | | 1993 | 6.7705979261911 | | 1992 | 6.991043730697 | | 1991 | 5.6264510386106 | | 1990 | 7.391907954001 |
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