| Year | Value | | 2021 | 28.615118151863 | | 2020 | 28.724431254624 | | 2019 | 28.833725156631 | | 2018 | 28.94300639027 | | 2017 | 29.060598182527 | | 2016 | 29.150777857242 | | 2015 | 29.26045570014 | | 2014 | 29.361471247802 | | 2013 | 29.460660365054 | | 2012 | 29.577291294766 | | 2011 | 29.678297627583 | | 2010 | 29.778849657754 | | 2009 | 29.866505211509 | | 2008 | 29.95413143658 | | 2007 | 30.041905600466 | | 2006 | 30.129680628834 | | 2005 | 30.058635910269 | | 2004 | 30.15696092341 | | 2003 | 30.230065318038 | | 2002 | 30.328310187673 | | 2001 | 30.426405320571 | | 2000 | 30.524500453469 | | 1999 | 30.624638896498 | | 1998 | 30.724777339526 | | 1997 | 30.824915782555 | | 1996 | 30.925054225583 | | 1995 | 31.025192668612 | | 1994 | 31.125331111641 | | 1993 | 31.118759366651 | | 1992 | 31.218555595704 | | 1991 | 30.874169932953 | | 1990 | 30.977763369255 |
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