| Year | Value | | 2020 | 1.6276164593566 | | 2019 | 1.5963223348541 | | 2018 | 1.5892072390162 | | 2017 | 1.5934554333881 | | 2016 | 1.5948122265504 | | 2015 | 1.6122740945068 | | 2014 | 1.5349263584635 | | 2013 | 1.4732290289179 | | 2012 | 1.5953450146859 | | 2011 | 1.6047221108166 | | 2010 | 1.7019413960002 | | 2009 | 1.7543609919908 | | 2008 | 1.7600092432799 | | 2007 | 1.7785228277607 | | 2006 | 1.7817291900619 | | 2005 | 1.8977341358151 | | 2004 | 1.8987146106578 | | 2003 | 1.9626858383607 | | 2002 | 2.0072502976994 | | 2001 | 2.0872188113297 | | 2000 | 2.1587744978417 | | 1999 | 2.2431390120912 | | 1998 | 2.4620744335735 | | 1997 | 2.5312747516513 | | 1996 | 2.384545769174 | | 1995 | 2.4466916207619 | | 1994 | 1.9436177923971 | | 1993 | 1.9225099340494 | | 1992 | 2.3545331092331 |
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