| Year | Value | | 2020 | 3.1960356981968 | | 2019 | 3.1381074692935 | | 2018 | 3.1576685748653 | | 2017 | 3.0999732892712 | | 2016 | 3.0727762803234 | | 2015 | 3.0906397676899 | | 2014 | 3.0711067269672 | | 2013 | 3.0784849348516 | | 2012 | 3.0112319872741 | | 2011 | 3.0015466007561 | | 2010 | 2.975915975073 | | 2009 | 2.8477203952015 | | 2008 | 3.23099183422 | | 2007 | 3.2350761016489 | | 2006 | 3.2281931542254 | | 2005 | 3.3904975252395 | | 2004 | 3.3845343802826 | | 2003 | 3.7330361156855 | | 2002 | 3.7714125269045 | | 2001 | 3.4217839319568 | | 2000 | 3.3957475336615 | | 1999 | 3.5439748561671 | | 1998 | 3.6198901316129 | | 1997 | 3.6006621061167 | | 1996 | 4.3299415457891 | | 1995 | 4.4985268327763 | | 1994 | 4.6808820054171 | | 1993 | 4.9576262606161 | | 1992 | 4.9037497654216 | | 1991 | 4.9739932940865 |
|