| Year | Value | | 2020 | 9.5934054839275 | | 2019 | 9.4597150404124 | | 2018 | 9.518165084377 | | 2017 | 9.6118010146502 | | 2016 | 9.709616373652 | | 2015 | 9.1697322764003 | | 2014 | 9.2596416270245 | | 2013 | 9.3407484018955 | | 2012 | 9.4247209131003 | | 2011 | 9.5114573401137 | | 2010 | 9.605708312323 | | 2009 | 9.7109243039436 | | 2008 | 9.2224223222786 | | 2007 | 9.3043507000177 | | 2006 | 9.4168045677101 | | 2005 | 9.5324169159694 | | 2004 | 9.6209244333963 | | 2003 | 9.5436691946502 | | 2002 | 9.6388758386144 | | 2001 | 9.7299558316655 | | 2000 | 9.6011338599499 | | 1999 | 9.7026511792387 | | 1998 | 9.7726315567857 | | 1997 | 9.8918071663757 | | 1996 | 9.4500934823619 | | 1995 | 10.196991436332 | | 1994 | 9.1789312486367 | | 1993 | 9.3402743459345 | | 1992 | 10.387006893077 | | 1991 | 10.61053833362 |
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