| Year | Value | | 2020 | 1.2792348227742 | | 2019 | 1.3111323997487 | | 2018 | 1.3569377889552 | | 2017 | 1.361647889552 | | 2016 | 1.3745333372659 | | 2015 | 1.3845688719149 | | 2014 | 1.4183821716623 | | 2013 | 1.4625977130391 | | 2012 | 1.4979804921784 | | 2011 | 1.537064032544 | | 2010 | 1.5699848373653 | | 2009 | 1.5864646214686 | | 2008 | 1.6468497534446 | | 2007 | 1.6944498176636 | | 2006 | 1.7471017418169 | | 2005 | 1.7798029354265 | | 2004 | 1.8178917737377 | | 2003 | 1.8510347508255 | | 2002 | 1.9064638499534 | | 2001 | 1.9646395178584 | | 2000 | 2.0261779033278 | | 1999 | 1.9603352718636 | | 1998 | 2.0386091552373 | | 1997 | 2.1026104135184 | | 1996 | 2.2163247045752 | | 1995 | 2.364853315221 | | 1994 | 1.6446645988696 | | 1993 | 1.6901875201857 | | 1992 | 1.7098045943923 | | 1991 | 1.7005415547645 |
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