| Year | Value | | 2022 | 3.609 | | 2021 | 4.045 | | 2020 | 4.25 | | 2019 | 3.419 | | 2018 | 3.709 | | 2017 | 4.156 | | 2016 | 5.115 | | 2015 | 6.813 | | 2014 | 7.725 | | 2013 | 10.177 | | 2012 | 11.003 | | 2011 | 11.029 | | 2010 | 11.172 | | 2009 | 10.03 | | 2008 | 7.817 | | 2007 | 7.406 | | 2006 | 7.494 | | 2005 | 7.189 | | 2004 | 5.832 | | 2003 | 5.79 | | 2002 | 5.608 | | 2001 | 5.671 | | 2000 | 6.563 | | 1999 | 6.929 | | 1998 | 8.925 | | 1997 | 8.991 | | 1996 | 10.022 | | 1995 | 10.168 | | 1994 | 10.847 | | 1993 | 12.098 | | 1992 | 9.942 | | 1991 | 8.5 | | 1990 | 1.7 |
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