| Year | Value | | 2021 | 6.3603181741368 | | 2020 | 6.7659454960833 | | 2019 | 6.0337099824805 | | 2018 | 5.9004287981987 | | 2017 | 5.3740335334143 | | 2014 | 5.0011865843239 | | 2013 | 5.0017059774229 | | 2012 | 5.0829916052907 | | 2011 | 5.2068622027078 | | 2010 | 5.243570056241 | | 2009 | 5.8114937013468 | | 2008 | 5.422454530631 | | 2007 | 5.4831419395412 | | 2006 | 5.5357152333053 | | 2005 | 5.8108957028113 | | 2004 | 5.870058870407 | | 2003 | 6.0175446858386 | | 2002 | 5.9821986058066 | | 2001 | 5.5896079624439 | | 2000 | 5.4791968412392 | | 1999 | 5.6675581629916 | | 1998 | 5.3814268911985 | | 1997 | 4.7635617253869 | | 1996 | 4.6183191581442 | | 1995 | 4.4394725825112 | | 1994 | 3.8667822426804 | | 1993 | 3.5647649354986 | | 1992 | 3.3213310397746 | | 1991 | 2.8335569815407 |
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