| Year | Value | | 2022 | 5.9875704022185 | | 2021 | 5.8519223098401 | | 2020 | 5.7543139037161 | | 2019 | 5.790665556437 | | 2018 | 5.6966360848999 | | 2017 | 5.5951741945237 | | 2016 | 5.5123866696397 | | 2015 | 5.4070212336546 | | 2014 | 5.3389209489787 | | 2013 | 7.9984608200644 | | 2012 | 11.609651899488 | | 2011 | 9.684920511711 | | 2010 | 9.6217019070774 | | 2009 | 9.4394061223785 | | 2008 | 9.3692144496905 | | 2007 | 9.1828203826162 | | 2006 | 9.1158185569919 | | 2005 | 9.0206297431729 | | 2004 | 8.9262240641257 | | 2003 | 8.9188771783526 | | 2002 | 8.8254721546391 | | 2001 | 8.7059346541284 | | 2000 | 8.5350546213351 | | 1999 | 8.5792780209367 | | 1998 | 8.5652133980235 | | 1997 | 8.4746958175913 | | 1996 | 8.5053472601064 | | 1995 | 8.5220533904363 | | 1994 | 8.5414594572297 | | 1993 | 8.5105906905535 | | 1992 | 8.5488953757023 | | 1991 | 8.5625445769811 |
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