| Year | Value | | 2019 | 2.83 | | 2018 | 2.87 | | 2017 | 3.06 | | 2016 | 3.13 | | 2015 | 3.12 | | 2014 | 3.16 | | 2013 | 3.22 | | 2012 | 3.25 | | 2011 | 3.29 | | 2010 | 3.58 | | 2009 | 3.7 | | 2008 | 3.91 | | 2007 | 4.12 | | 2006 | 5.3391 | | 2002 | 7.5 | | 1995 | 14.6000003815 | | 1994 | 14.6999998093 | | 1993 | 14.8000001907 | | 1992 | 15.8999996185 | | 1991 | 16.2999992371 | | 1990 | 16.7000007629 | | 1985 | 15.800000190735 | | 1980 | 14.800000190735 | | 1970 | 12.89999961853 | | 1960 | 9.8000001907349 |
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