| Year | Value | | 2016 | 0.83 | | 2015 | 0.78 | | 2014 | 0.83 | | 2011 | 1.36 | | 2010 | 1.3 | | 2009 | 1.3 | | 2008 | 1.3 | | 2007 | 1.3 | | 2006 | 1.3 | | 2005 | 1.2 | | 2002 | 1.2000000477 | | 1996 | 1.3400000334 | | 1994 | 0.6008999944 | | 1993 | 1.2999999523 | | 1990 | 0.9322000146 | | 1989 | 1.3200999498367 | | 1975 | 1.6829999685288 | | 1970 | 1.6294000148773 | | 1960 | 2.3892507553101 |
|