| Year | Value | | 2022 | 3.256 | | 2021 | 4.019 | | 2020 | 4.441 | | 2019 | 3.477 | | 2018 | 3.275 | | 2017 | 3.419 | | 2016 | 3.853 | | 2015 | 4.311 | | 2014 | 4.819 | | 2013 | 4.921 | | 2012 | 4.892 | | 2011 | 5.175 | | 2010 | 5.303 | | 2009 | 5.356 | | 2008 | 3.874 | | 2007 | 3.628 | | 2006 | 3.566 | | 2005 | 3.558 | | 2004 | 3.937 | | 2003 | 3.455 | | 2002 | 3.003 | | 2001 | 2.627 | | 2000 | 2.646 | | 1999 | 2.598 | | 1998 | 3.733 | | 1997 | 4.235 | | 1996 | 5.465 | | 1995 | 7.095 | | 1994 | 4.436 | | 1993 | 3.365 | | 1992 | 3.235 | | 1991 | 3.145 | | 1988 | 1.76 |
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