| Year | Value | | 2022 | 18.150859233699 | | 2021 | 17.744936470597 | | 2020 | 17.397370810385 | | 2019 | 17.435323573335 | | 2018 | 16.554658943759 | | 2017 | 15.553390654934 | | 2016 | 14.519999571073 | | 2015 | 13.650911001921 | | 2014 | 13.020687500165 | | 2013 | 12.388812297041 | | 2012 | 11.75828962435 | | 2011 | 11.032656663725 | | 2010 | 10.497984742968 | | 2009 | 9.9436609141785 | | 2008 | 9.6011356341728 | | 2007 | 9.0178861649671 | | 2006 | 8.5087035463327 | | 2005 | 8.0992255189969 | | 2004 | 7.6389908871228 | | 2003 | 7.1810435254858 | | 2002 | 6.946980739053 | | 2001 | 6.7745393805416 | | 2000 | 6.5934148113328 | | 1999 | 6.4351476014427 | | 1998 | 6.3548849065263 | | 1997 | 6.256130623271 | | 1996 | 6.1765773620558 | | 1995 | 6.1280547222508 | | 1994 | 6.085469275824 | | 1993 | 6.0345800430814 | | 1992 | 6.0173622232369 | | 1991 | 5.9948085903816 |
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