| Year | Value | | 2017 | 2.96 | | 2015 | 2.83 | | 2014 | 2.76 | | 2013 | 2.85 | | 2012 | 2.86 | | 2011 | 2.9 | | 2010 | 2.92 | | 2009 | 3.2 | | 2008 | 3.2 | | 2007 | 3.2 | | 2006 | 3.2 | | 2004 | 3.4 | | 2003 | 3.4 | | 2002 | 3.4000000954 | | 2001 | 3.4 | | 2000 | 3.5 | | 1996 | 3.9400000572 | | 1995 | 3.6 | | 1993 | 3.8838999271 | | 1990 | 4 | | 1980 | 4.4047999382019 | | 1970 | 5.0935997962952 | | 1960 | 5.7256636619568 |
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