| Year | Value | | 2017 | 5.97 | | 2016 | 5.89 | | 2015 | 5.9 | | 2014 | 5.93 | | 2013 | 6.34 | | 2012 | 6.34 | | 2010 | 6.76 | | 2009 | 6.8 | | 2008 | 7.6 | | 2006 | 6.6 | | 2005 | 6.7 | | 2004 | 7.3 | | 2003 | 7.3 | | 1996 | 7.5599999428 | | 1995 | 7.4 | | 1993 | 1.2676000595 | | 1990 | 8.3999996185 | | 1980 | 8.5347003936768 | | 1970 | 10.221599578857 | | 1960 | 5.9297790527344 |
|