| Year | Value | | 2022 | 9.1155958706234 | | 2021 | 8.924054584127 | | 2020 | 8.7831388628581 | | 2019 | 8.8253661243496 | | 2018 | 8.8731199305885 | | 2017 | 8.7268932981443 | | 2016 | 8.8474466246993 | | 2015 | 8.8604016686085 | | 2014 | 8.6562022028508 | | 2013 | 8.4954120280747 | | 2012 | 8.419355097795 | | 2011 | 8.2544380697522 | | 2010 | 8.1133037074827 | | 2009 | 7.8729746609753 | | 2008 | 7.7557237085297 | | 2007 | 7.520029711179 | | 2006 | 7.2408557444945 | | 2005 | 6.8030405409362 | | 2004 | 6.4045200098029 | | 2003 | 5.9489966409359 | | 2002 | 5.7233858529254 | | 2001 | 5.581037924219 | | 2000 | 5.4519834799666 | | 1999 | 5.4142080894767 | | 1998 | 5.3481150326854 | | 1997 | 5.2085274157684 | | 1996 | 5.1383520171621 | | 1995 | 5.0897365096297 | | 1994 | 5.0573693477033 | | 1993 | 4.9910108039145 | | 1992 | 5.0924989743603 | | 1991 | 4.9965518173361 |
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