| Year | Value | | 2017 | 2.49 | | 2016 | 2.48 | | 2015 | 2.4 | | 2014 | 2.24 | | 2013 | 2.19 | | 2011 | 2 | | 2008 | 3.1 | | 2007 | 3.22 | | 2006 | 3.2 | | 2005 | 3.2 | | 2003 | 2.8 | | 2001 | 2.9000000954 | | 1994 | 3.5652000904 | | 1993 | 3.6426999569 | | 1992 | 3.4514000416 | | 1991 | 3.5471999645 | | 1990 | 3.6078000069 | | 1981 | 3.9079999923706 | | 1980 | 3.9565000534058 | | 1975 | 3.333300113678 | | 1970 | 3.6982998847961 | | 1960 | 4.392466545105 |
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