| Year | Value | | 2022 | 14.845260927464 | | 2021 | 14.519399312306 | | 2020 | 14.071183508006 | | 2019 | 14.084253772549 | | 2018 | 13.739914393899 | | 2017 | 13.410215502654 | | 2016 | 13.064174909972 | | 2015 | 12.622372662319 | | 2014 | 12.201540985581 | | 2013 | 11.771412306725 | | 2012 | 11.380061925787 | | 2011 | 10.887951578673 | | 2010 | 10.52170042337 | | 2009 | 10.202219999048 | | 2008 | 9.8018046893877 | | 2007 | 9.4020698596048 | | 2006 | 9.0591227206329 | | 2005 | 8.7676762618338 | | 2004 | 8.885841430362 | | 2003 | 8.9351862865753 | | 2002 | 9.2145254721427 | | 2001 | 9.4216417415847 | | 2000 | 9.5344027540649 | | 1999 | 9.717218230733 | | 1998 | 9.7433100251414 | | 1997 | 9.7978556882717 | | 1996 | 9.6677282519652 | | 1995 | 9.5593285142937 | | 1994 | 9.5484680814792 | | 1993 | 9.6314650269702 | | 1992 | 9.6943937841506 | | 1991 | 9.8086072079868 |
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