| Year | Value | | 2022 | 4.5972252372208 | | 2021 | 5.9036922521756 | | 2020 | 6.9054238699918 | | 2019 | 4.9748691582204 | | 2018 | 5.2984181320962 | | 2017 | 5.836243843317 | | 2016 | 6.4042874207747 | | 2015 | 6.8224491090305 | | 2014 | 7.3906946100578 | | 2013 | 7.9149010300419 | | 2012 | 8.0293366093298 | | 2011 | 7.9729270932702 | | 2010 | 8.3373113711932 | | 2009 | 8.0398336348512 | | 2008 | 5.830129104372 | | 2007 | 5.4864853457824 | | 2006 | 5.8991896764207 | | 2005 | 6.2390691413085 | | 2004 | 6.5185389658611 | | 2003 | 6.7123324151992 | | 2002 | 6.6310491488763 | | 2001 | 6.4238434171958 | | 2000 | 6.3999007771176 | | 1999 | 6.9644590071649 | | 1998 | 7.2399714000507 | | 1997 | 7.1834687694792 | | 1996 | 7.3214078058548 | | 1995 | 7.203128412196 | | 1994 | 7.6462019039684 | | 1993 | 7.6497865025846 | | 1992 | 7.1154932910927 | | 1991 | 6.6867940288474 |
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