| Year | Value | | 2021 | 6.7122002085506 | | 2020 | 6.7122002085506 | | 2019 | 6.7122002085506 | | 2018 | 6.7153284671533 | | 2017 | 6.7142857142857 | | 2016 | 6.7142857142857 | | 2015 | 6.6767466110532 | | 2014 | 6.6767466110532 | | 2013 | 6.6668404588113 | | 2012 | 6.6558915537018 | | 2011 | 6.6522419186653 | | 2010 | 6.6538060479666 | | 2009 | 6.6527632950991 | | 2008 | 6.6710114702815 | | 2007 | 6.6736183524505 | | 2006 | 6.6929092805005 | | 2005 | 6.6944734098019 | | 2004 | 6.9551616266945 | | 2003 | 7.007299270073 | | 2002 | 7.0125130344108 | | 2001 | 7.007299270073 | | 2000 | 7.0698644421272 | | 1999 | 7.1324296141814 | | 1998 | 7.0907194994786 | | 1997 | 7.0802919708029 | | 1996 | 7.0750782064651 | | 1995 | 7.0698644421272 | | 1994 | 7.0594369134515 | | 1993 | 7.0490093847758 | | 1992 | 6.8821689259646 |
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