| Year | Value | | 2010 | 8.61 | | 2009 | 8.14 | | 2008 | 10.94 | | 2007 | 8.42 | | 2006 | 8.17 | | 2003 | 10.28 | | 2002 | 11.68 | | 2001 | 11.63 | | 1999 | 7.6 | | 1995 | 7.2 | | 1994 | 7.5 | | 1993 | 7.4 | | 1991 | 6.75 | | 1989 | 5.78 | | 1984 | 4.7 | | 1983 | 4.24 | | 1979 | 3.8 | | 1978 | 4.5 | | 1977 | 5 | | 1976 | 6.2 | | 1975 | 4.81 | | 1974 | 5.1 | | 1973 | 4.5 | | 1972 | 4.7 | | 1971 | 7.5 | | 1970 | 6.4 | | 1969 | 4.3 |
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