| Year | Value | | 2020 | 7.53 | | 2017 | 5.045 | | 2016 | 5.319 | | 2012 | 6.601 | | 2011 | 6.902 | | 2010 | 7.112 | | 2009 | 6.097 | | 2008 | 6.245 | | 2007 | 7.279 | | 2006 | 8.606 | | 2005 | 10.663 | | 2003 | 16.78 | | 2002 | 16.17 | | 2001 | 8.83 | | 1996 | 11.77 | | 1995 | 10.24 | | 1994 | 8.56 | | 1990 | 13.96 | | 1989 | 9.74 | | 1988 | 7.28 | | 1987 | 9.1 | | 1986 | 10.93 | | 1985 | 13.16 | | 1984 | 12.86 | | 1983 | 11.49 | | 1982 | 8.05 | | 1981 | 7 | | 1980 | 5.89 | | 1979 | 5.81 | | 1978 | 4.98 | | 1977 | 5.11 | | 1976 | 6.34 | | 1975 | 7.01 |
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