| Year | Value | | 2022 | 4.013 | | 2021 | 4.747 | | 2020 | 4.974 | | 2019 | 4.448 | | 2018 | 5.116 | | 2017 | 6.573 | | 2016 | 8.007 | | 2015 | 8.975 | | 2014 | 9.692 | | 2013 | 10.116 | | 2012 | 8.852 | | 2011 | 8.182 | | 2010 | 7.248 | | 2009 | 5.869 | | 2008 | 4.379 | | 2007 | 4.828 | | 2006 | 5.957 | | 2005 | 6.515 | | 2004 | 6.017 | | 2003 | 6.481 | | 2002 | 5.927 | | 2001 | 5.685 | | 2000 | 6.92 | | 1999 | 7.33 | | 1998 | 7.4 | | 1997 | 6.658 | | 1996 | 6.886 | | 1995 | 7.15 | | 1994 | 8.24 | | 1993 | 8.51 | | 1991 | 7.08 |
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