| Year | Value | | 2020 | 1.5150264711007 | | 2019 | 1.4218427980669 | | 2018 | 1.4224386325349 | | 2017 | 1.4267587476738 | | 2016 | 1.4221883994635 | | 2015 | 1.4265022580303 | | 2014 | 1.4358381006011 | | 2013 | 1.442004856596 | | 2012 | 1.4501271502216 | | 2011 | 1.4581572600395 | | 2010 | 1.4780180541373 | | 2009 | 1.4665795654379 | | 2008 | 1.5161278095744 | | 2007 | 1.5277954233682 | | 2006 | 1.5449029079342 | | 2005 | 1.5694674156881 | | 2004 | 1.5737259115021 | | 2003 | 1.5397341675507 | | 2002 | 1.5554471867753 | | 2001 | 1.5677797686779 | | 2000 | 1.8442062569008 | | 1999 | 2.0194124017523 | | 1998 | 1.7627904392494 | | 1997 | 1.7724581099718 | | 1996 | 2.6862314610526 | | 1995 | 2.8249861826281 | | 1994 | 3.2072178896781 | | 1993 | 4.0307739224129 | | 1992 | 4.0036183558809 | | 1991 | 6.7838793099904 |
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