| Year | Value | | 2020 | 4.4532651650658 | | 2019 | 3.2341768844368 | | 2018 | 3.2571846853768 | | 2017 | 2.0856963741017 | | 2016 | 2.0846986114311 | | 2015 | 2.1526259152265 | | 2014 | 2.2267448463642 | | 2013 | 2.0118565867969 | | 2012 | 9.7769398007839 | | 2011 | 10.215546243361 | | 2010 | 10.57327885758 | | 2009 | 8.9748039383818 | | 2008 | 8.1523820073397 | | 2007 | 5.2218861654781 | | 2006 | 7.1039066176406 | | 2005 | 3.2912469360231 | | 2004 | 2.6800186493365 | | 2003 | 6.6693004228059 | | 2002 | 6.8881911052087 | | 2001 | 7.70205991345 | | 2000 | 8.1676625019545 | | 1999 | 8.4565745188403 | | 1998 | 8.7481490085553 | | 1997 | 8.358300666324 | | 1996 | 8.4153039062965 | | 1995 | 8.2052997009387 | | 1994 | 8.8682227998044 | | 1993 | 8.9349398077034 | | 1992 | 9.4068224193007 | | 1991 | 11.290397338794 |
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