| Year | Value | | 2020 | 6.1962994699262 | | 2019 | 6.6617413217448 | | 2018 | 7.519920423209 | | 2017 | 7.6039292287321 | | 2016 | 8.0870655332993 | | 2015 | 8.3513382670135 | | 2014 | 8.8119553931564 | | 2013 | 8.7647253770064 | | 2012 | 9.3775947150544 | | 2011 | 9.2093586170192 | | 2010 | 8.1634410798229 | | 2009 | 6.5811813810255 | | 2008 | 7.8141442540804 | | 2007 | 7.8125296661611 | | 2006 | 8.2628458548156 | | 2005 | 8.7244163845774 | | 2004 | 9.6807622028763 | | 2003 | 9.1168955068118 | | 2002 | 9.709831678852 | | 2001 | 8.0838716726408 | | 1999 | 10.884818978344 | | 1998 | 11.262145376098 | | 1997 | 10.52276441094 | | 1996 | 8.9524883881313 | | 1995 | 8.4295215264988 | | 1994 | 7.6782509361674 | | 1993 | 5.9119103095792 | | 1992 | 5.6007810025562 | | 1991 | 7.4506083243191 | | 1990 | 8.6391896656738 |
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