| Year | Value | | 2021 | 4.062 | | 2020 | 3.484 | | 2019 | 3.343 | | 2018 | 3.302 | | 2017 | 3.282 | | 2016 | 3.219 | | 2015 | 3.143 | | 2014 | 3.035 | | 2013 | 3.958 | | 2012 | 3.98 | | 2011 | 4.136 | | 2010 | 4.16 | | 2009 | 4.034 | | 2008 | 3.981 | | 2007 | 3.867 | | 2006 | 3.66 | | 2005 | 3.56 | | 2004 | 2.763 | | 2003 | 2.594 | | 2002 | 2.425 | | 2001 | 2.394 | | 2000 | 2.239 | | 1999 | 2.277 | | 1998 | 2.202 | | 1997 | 2.143 | | 1996 | 2.113 | | 1995 | 2.113 | | 1994 | 2.003 | | 1993 | 2.034 | | 1992 | 2.011 | | 1991 | 1.711 | | 1990 | 1.564 |
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