| Year | Value | | 2022 | 6.7458206686342 | | 2021 | 7.7300669403387 | | 2020 | 7.8233478181366 | | 2019 | 7.5060232631879 | | 2018 | 8.1582011906028 | | 2017 | 9.0533717046465 | | 2016 | 10.028635929348 | | 2015 | 10.890155505659 | | 2014 | 11.626841119911 | | 2013 | 11.974675199028 | | 2012 | 11.418597851705 | | 2011 | 10.235179538001 | | 2010 | 10.157236447988 | | 2009 | 9.5997277671125 | | 2008 | 7.5538604659044 | | 2007 | 7.5263705129439 | | 2006 | 8.4111973372023 | | 2005 | 9.1337197109554 | | 2004 | 9.2944444408093 | | 2003 | 9.0295902784535 | | 2002 | 8.697689707455 | | 2001 | 8.7279257623746 | | 2000 | 9.5300641464784 | | 1999 | 10.489987764736 | | 1998 | 11.241807349274 | | 1997 | 11.660642643263 | | 1996 | 11.660364379689 | | 1995 | 11.452704042255 | | 1994 | 11.772551725996 | | 1993 | 10.622490077757 | | 1992 | 8.994716772177 | | 1991 | 8.8824464992066 |
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