| Year | Value | | 2017 | 0.87 | | 2016 | 0.86 | | 2015 | 0.73 | | 2014 | 0.78 | | 2011 | 0.93 | | 2009 | 1.1 | | 2007 | 1.3 | | 2003 | 0.9 | | 2001 | 0.8000000119 | | 1996 | 1.4700000286 | | 1993 | 2.5999999046 | | 1992 | 2.5729999542 | | 1991 | 2.6775999069 | | 1990 | 2.6779000759 | | 1988 | 2.4995999336243 | | 1987 | 2.5982999801636 | | 1986 | 2.7000999450684 | | 1985 | 2.6930999755859 | | 1984 | 0.37279999256134 | | 1983 | 0.35820001363754 | | 1982 | 0.37040001153946 | | 1980 | 0.3319000005722 | | 1976 | 0.34290000796318 | | 1975 | 0.33149999380112 | | 1974 | 0.34299999475479 |
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