| Year | Value | | 2018 | 6.4226670753303 | | 2017 | 6.5931836148694 | | 2016 | 6.5830209177252 | | 2015 | 6.5796501696732 | | 2014 | 6.5822227269859 | | 2013 | 6.5563545475167 | | 2012 | 6.574336637523 | | 2011 | 6.5424587178813 | | 2010 | 6.5691599136718 | | 2009 | 6.6548091499585 | | 2008 | 6.6646369144637 | | 2007 | 6.6033108985125 | | 2006 | 6.7339566956929 | | 2005 | 6.774348965948 | | 2004 | 6.8275099013732 | | 2003 | 6.8878742634397 | | 2002 | 6.7378232465484 | | 2000 | 6.6963522846698 | | 1999 | 6.6165306786963 | | 1998 | 6.7973288388091 | | 1997 | 7.0230654493653 | | 1996 | 7.2682465567856 | | 1995 | 7.2386886995594 | | 1994 | 7.6306715967644 | | 1993 | 7.6428860583175 | | 1992 | 7.6722857567347 | | 1991 | 8.0388287647444 | | 1990 | 8.1198405040174 | | 1985 | 8.2004950903895 | | 1980 | 8.0302452107074 |
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