| Year | Value | | 2020 | 1.9242903545507 | | 2019 | 2.1425613548339 | | 2018 | 2.1013991877703 | | 2017 | 2.1371104225792 | | 2016 | 2.1174266003815 | | 2015 | 2.09880988381 | | 2014 | 2.1159248980852 | | 2013 | 2.1330464058636 | | 2012 | 2.1505720563571 | | 2011 | 2.1698483287644 | | 2010 | 2.1909722721733 | | 2009 | 2.2045470926051 | | 2008 | 2.2121279808748 | | 2007 | 2.2208962264376 | | 2006 | 2.2294398708333 | | 2005 | 2.1057786227632 | | 2004 | 2.1091617261913 | | 2003 | 2.6112862160163 | | 2002 | 2.3979673272876 | | 2001 | 1.9700555983123 | | 2000 | 1.9188049909678 | | 1999 | 1.9344303626711 | | 1998 | 1.98361706105 | | 1997 | 1.9891206389961 | | 1996 | 1.9143334147816 | | 1995 | 1.776142771838 | | 1994 | 1.5631136252537 | | 1993 | 1.5855403447634 | | 1992 | 1.4299825542128 | | 1991 | 1.4481722445941 |
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