| Year | Value | | 2020 | 1.9962565436814 | | 2019 | 2.0413460052317 | | 2018 | 2.0884856826043 | | 2017 | 2.1666460319426 | | 2016 | 2.1918505210507 | | 2015 | 2.2322958064958 | | 2014 | 2.2724671800154 | | 2013 | 2.3126426684236 | | 2012 | 2.3478619254495 | | 2011 | 2.3950981473313 | | 2010 | 2.4487976016568 | | 2009 | 2.5052277272411 | | 2008 | 2.5832990943692 | | 2007 | 2.6444865603415 | | 2006 | 2.6999610434192 | | 2005 | 2.7538058252173 | | 2004 | 2.6731499463365 | | 2003 | 2.8184048645396 | | 2002 | 2.8695597928926 | | 2001 | 2.9049639629349 | | 2000 | 2.9831688747401 | | 1999 | 3.0688933110015 | | 1998 | 3.159593919503 | | 1997 | 3.2472842325087 | | 1996 | 3.3222751325299 | | 1995 | 3.3720329775051 | | 1994 | 1.8387300059006 | | 1993 | 2.7727569695846 | | 1992 | 2.9076318065843 | | 1991 | 3.2319207304901 |
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