| Year | Value | | 2020 | 1.2877920364974 | | 2019 | 1.32403111039 | | 2018 | 1.3488997691306 | | 2017 | 1.3750804760213 | | 2016 | 1.4074585605059 | | 2015 | 1.2777090379265 | | 2014 | 1.2923686135205 | | 2013 | 1.2745519427367 | | 2012 | 1.2924185219044 | | 2011 | 1.2758654825815 | | 2010 | 1.3242669289057 | | 2009 | 1.3525816829854 | | 2008 | 1.3597803123085 | | 2007 | 1.4113645264077 | | 2006 | 1.4476600613077 | | 2005 | 1.6936299366028 | | 2004 | 1.7347841113516 | | 2003 | 1.748776929019 | | 2002 | 1.8478068753538 | | 2001 | 1.9420185105195 | | 2000 | 1.8545589596131 | | 1999 | 1.9592376214567 | | 1998 | 2.0149656700224 | | 1997 | 2.080588917739 | | 1996 | 2.0399878106932 | | 1995 | 2.256364292445 | | 1994 | 1.8290777090838 | | 1993 | 1.6924168503644 | | 1992 | 1.7420329448726 | | 1991 | 1.7703905717653 |
|