| Year | Value | | 2021 | 4.4869995603653 | | 2020 | 4.1311550769929 | | 2019 | 4.3512325437691 | | 2018 | 4.3934844700635 | | 2017 | 4.352136471477 | | 2016 | 4.4072480709799 | | 2015 | 4.3489731371472 | | 2014 | 4.2100722131919 | | 2013 | 4.250752251598 | | 2012 | 5.4011519092372 | | 2011 | 6.2985351452326 | | 2010 | 6.8117165088692 | | 2009 | 6.7195918129622 | | 2008 | 7.2766518767238 | | 2007 | 8.4073039225004 | | 2006 | 9.5535674969543 | | 2005 | 15.95792212345 | | 2004 | 11.031705773909 | | 2003 | 11.733437807307 | | 2002 | 11.721060564467 | | 2001 | 12.39509114231 | | 2000 | 11.981674142095 | | 1999 | 13.085566967178 | | 1998 | 14.472039376047 | | 1997 | 16.387475792622 | | 1996 | 18.438113948919 | | 1995 | 24.567820616445 | | 1994 | 23.880638379583 | | 1993 | 22.571764691831 | | 1992 | 25.563016897191 | | 1991 | 19.106723382642 | | 1990 | 22.199725798305 |
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