| Year | Value | | 2017 | 0.44 | | 2015 | 0.58 | | 2014 | 0.62 | | 2012 | 0.61 | | 2011 | 0.66 | | 2010 | 0.6 | | 2009 | 0.6 | | 2008 | 0.6 | | 2007 | 0.7 | | 2005 | 0.7 | | 2002 | 0.5 | | 1996 | 0.9800000191 | | 1993 | 1.1000000238 | | 1990 | 1.1000000238 | | 1989 | 0.87779998779297 | | 1986 | 1.7211999893189 | | 1975 | 2.0027000904083 | | 1970 | 2.1710999011993 | | 1960 | 2.6310875415802 |
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