| Year | Value | | 2020 | 1.4368569293492 | | 2019 | 1.4043282958443 | | 2018 | 1.4068143821768 | | 2017 | 1.3914718237999 | | 2016 | 1.6735359784998 | | 2015 | 1.6511909599051 | | 2014 | 1.643599781172 | | 2013 | 1.6489458571407 | | 2012 | 1.641376842874 | | 2011 | 1.6493378992995 | | 2010 | 1.6447231229464 | | 2009 | 1.6238748419708 | | 2008 | 1.6220421319613 | | 2007 | 1.5876252395057 | | 2006 | 1.5699576172611 | | 2005 | 1.8316617953431 | | 2004 | 1.7637134987717 | | 2003 | 1.7652382623118 | | 2002 | 1.7283190529082 | | 2001 | 1.5617030262383 | | 2000 | 2.4046405738089 | | 1999 | 2.3949267172742 | | 1998 | 2.5028776105975 | | 1997 | 2.5614685496355 | | 1996 | 2.6273886166658 | | 1995 | 2.4531528474734 | | 1994 | 1.65748763825 | | 1993 | 1.6631611387764 | | 1992 | 1.4379706689097 | | 1991 | 1.6860943603835 |
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