| Year | Value | | 2021 | 7.3750282593627 | | 2020 | 7.7627494690214 | | 2019 | 10.903951332927 | | 2018 | 11.153760646457 | | 2017 | 11.608682738476 | | 2016 | 11.423489767796 | | 2015 | 11.720264857588 | | 2014 | 9.7478598226756 | | 2013 | 10.121257054083 | | 2012 | 10.111030705252 | | 2011 | 10.847604911487 | | 2010 | 10.931868686571 | | 2009 | 12.80032047696 | | 2008 | 13.277334434901 | | 2007 | 14.221608589517 | | 2006 | 14.57725917759 | | 2005 | 13.732529022147 | | 2004 | 13.465968956989 | | 2003 | 12.710675852745 | | 2002 | 14.020361783568 | | 2001 | 14.625500391503 | | 2000 | 14.502765506076 | | 1999 | 15.012075449179 | | 1998 | 14.476426984261 | | 1997 | 16.0076920312 | | 1996 | 16.950690509915 | | 1995 | 17.752017155556 | | 1994 | 17.167975630479 | | 1993 | 17.469998899042 | | 1992 | 17.953456874599 | | 1991 | 18.304797969625 | | 1990 | 19.020833851279 |
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