| Year | Value | | 2022 | 5.614 | | 2021 | 5.59 | | 2020 | 5.035 | | 2019 | 3.912 | | 2018 | 4.187 | | 2017 | 4.927 | | 2016 | 5.901 | | 2015 | 6.812 | | 2014 | 6.802 | | 2013 | 7.096 | | 2012 | 6.794 | | 2011 | 7.177 | | 2010 | 6.959 | | 2009 | 6.859 | | 2008 | 5.788 | | 2007 | 6.413 | | 2006 | 7.27 | | 2005 | 7.174 | | 2004 | 7.719 | | 2003 | 6.95 | | 2002 | 11.75 | | 2001 | 6.562 | | 2000 | 6.969 | | 1999 | 6.239 | | 1998 | 5.63 | | 1997 | 5.515 | | 1996 | 6.74 | | 1995 | 8.01 | | 1994 | 8.17 | | 1992 | 8.26 |
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