Year | Value | 2017 | 2.4898628725157 | 2016 | 2.3854490384306 | 2015 | 2.3457559680444 | 2014 | 2.3512357631516 | 2013 | 2.3385739633722 | 2012 | 2.236393754942 | 2011 | 2.1624553285815 | 2010 | 2.0821673668746 | 2009 | 2.0100044151272 | 2008 | 2.0512400730608 | 2007 | 2.0125008731021 | 2006 | 2.0417087991866 | 2005 | 2.0758136929635 | 2003 | 2.1566644942461 | 2002 | 1.9848044475923 | 2001 | 2.0578839154864 | 2000 | 2.1833850178125 | 1991 | 2.801535125136 | 1990 | 3.4938635472596 | 1987 | 3.066006145558 | 1985 | 2.8679974220616 | 1981 | 2.2195323123057 | 1980 | 2.2731615662847 | 1975 | 1.6071557595198 | 1970 | 1.5317396870293 |
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