| Year | Value | | 2020 | 3.7485828996743 | | 2019 | 3.6579331685225 | | 2018 | 3.7332086575111 | | 2017 | 3.6167380354427 | | 2016 | 3.5240821711579 | | 2015 | 3.0873198117573 | | 2014 | 2.7197128100302 | | 2013 | 2.7139204289722 | | 2012 | 2.7045761525483 | | 2011 | 2.7125543483547 | | 2010 | 2.7195284817888 | | 2009 | 2.735346798694 | | 2008 | 2.6278576100906 | | 2007 | 2.646397005024 | | 2006 | 2.6661986841245 | | 2005 | 2.5211820259387 | | 2004 | 3.0330391877546 | | 2003 | 3.0793427360874 | | 2002 | 3.176164924522 | | 2001 | 2.7244846681445 | | 2000 | 2.6654241984647 | | 1999 | 2.8000027191964 | | 1998 | 2.847811055071 | | 1997 | 3.0988686401586 | | 1996 | 3.1087938418273 | | 1995 | 3.3370794589085 | | 1994 | 1.5270965230511 | | 1993 | 1.5654167004061 | | 1992 | 1.5983734951313 | | 1991 | 1.5615539733386 |
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