| Year | Value | | 2017 | 3.5887112960868 | | 2016 | 3.4263318140132 | | 2015 | 3.3009447616368 | | 2014 | 3.2788471230654 | | 2013 | 3.1790319941306 | | 2012 | 2.9970026185347 | | 2011 | 2.8090376084597 | | 2010 | 2.682158470082 | | 2009 | 2.6285402656588 | | 2008 | 2.7801905258308 | | 2007 | 2.7032493030095 | | 2006 | 2.7304584698639 | | 2005 | 2.7300619867893 | | 2004 | 2.6851148530491 | | 2003 | 2.6310379498591 | | 2002 | 2.4596374888138 | | 2001 | 2.6939345569756 | | 2000 | 2.75328396684 | | 1999 | 3.3469248369848 | | 1998 | 3.0999831372206 | | 1997 | 3.5222006826425 | | 1996 | 3.4145545211542 | | 1995 | 3.6160556139094 | | 1994 | 3.3593998296709 | | 1993 | 3.3851278790538 | | 1992 | 3.3814201424473 | | 1991 | 3.4442971044648 | | 1990 | 3.3804325236997 | | 1989 | 3.5805603057893 | | 1988 | 4.0621464618589 | | 1987 | 3.959122038162 | | 1986 | 3.9409260080162 | | 1985 | 3.5117871828477 | | 1980 | 2.4948123182439 | | 1975 | 2.0802127898952 | | 1970 | 1.8033880141401 |
|