| Year | Value | | 2022 | 6.6660975503733 | | 2021 | 6.5109107193194 | | 2020 | 6.3972386148042 | | 2019 | 6.4541182014963 | | 2018 | 6.3604018682663 | | 2017 | 6.3332693561726 | | 2016 | 6.3354976350049 | | 2015 | 6.4127920334242 | | 2014 | 6.5831423250916 | | 2013 | 6.8895615674136 | | 2012 | 7.527262557214 | | 2011 | 7.2729692009273 | | 2010 | 7.0748501937508 | | 2009 | 6.9569117009935 | | 2008 | 6.8466900251617 | | 2007 | 6.799910363408 | | 2006 | 6.7253795307479 | | 2005 | 6.6813432345403 | | 2004 | 6.6911736029769 | | 2003 | 6.672312974087 | | 2002 | 6.6219314771779 | | 2001 | 6.5465325832053 | | 2000 | 6.4552630391743 | | 1999 | 6.4262910769498 | | 1998 | 6.4678436179245 | | 1997 | 6.4270753483247 | | 1996 | 6.4090523799265 | | 1995 | 6.4235442252939 | | 1994 | 6.395895336703 | | 1993 | 6.435368611488 | | 1992 | 6.4806369961981 | | 1991 | 6.5314868913535 |
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