| Year | Value | | 2021 | 6.6601428084407 | | 2020 | 6.4271720466828 | | 2019 | 6.805558051262 | | 2018 | 6.9099995316824 | | 2017 | 5.7944643444986 | | 2016 | 5.5007123885279 | | 2015 | 4.6896816290536 | | 2014 | 5.0536908546784 | | 2013 | 5.4849895050288 | | 2012 | 4.719876330466 | | 2011 | 5.1549683177897 | | 2010 | 5.1985727241613 | | 2009 | 4.8428929746408 | | 2008 | 5.7304610272089 | | 2007 | 6.0804148363089 | | 2006 | 5.3958492632903 | | 2005 | 4.9145170376387 | | 2004 | 4.9361941276273 | | 2003 | 4.7013019956958 | | 2002 | 4.0567442436327 | | 2001 | 5.9906528255048 | | 2000 | 6.9511989995539 | | 1999 | 8.2959946216711 | | 1998 | 9.6201003089278 | | 1997 | 10.335288824623 | | 1996 | 8.6701589514153 | | 1995 | 7.6799083857805 | | 1994 | 8.066651689475 | | 1993 | 8.4969561409987 | | 1992 | 11.526440109117 | | 1991 | 6.6138325349346 | | 1990 | 4.9748110831234 |
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