| Year | Value | | 2010 | 4.0561417360959 | | 2009 | 4.0277077585694 | | 2008 | 3.5523398057458 | | 2007 | 4.1005436533306 | | 2006 | 4.3944468231113 | | 2005 | 5.0264199038793 | | 2004 | 5.5418386758124 | | 2003 | 6.2483242719893 | | 2002 | 5.412501222733 | | 2001 | 5.480550467758 | | 2000 | 5.4536564211942 | | 1999 | 5.8105804964522 | | 1998 | 5.8083811123875 | | 1997 | 5.8827553184212 | | 1996 | 6.0419160549868 | | 1995 | 6.9496913174832 | | 1994 | 7.3641427304036 | | 1993 | 7.2380998416938 | | 1992 | 8.990662755967 | | 1991 | 10.425787318175 | | 1990 | 6.8680868191169 | | 1989 | 7.9725408344982 | | 1988 | 7.8539293834354 | | 1987 | 11.218209721874 | | 1986 | 14.449681286462 | | 1985 | 16.555121658156 | | 1984 | 17.686018422659 | | 1983 | 14.638905186176 | | 1982 | 15.559399895331 | | 1981 | 14.546117943962 | | 1980 | 17.249853715623 | | 1979 | 15.928567763124 | | 1978 | 14.705609929297 | | 1977 | 14.504127642246 | | 1976 | 14.725985844287 | | 1975 | 15.94892460067 | | 1974 | 12.660145156201 | | 1973 | 15.069465571443 | | 1972 | 10.771564802774 | | 1971 | 8.436291027081 | | 1970 | 11.220588235294 | | 1969 | 10.826169313711 | | 1968 | 12.164879356568 | | 1967 | 8.2139619220308 | | 1966 | 7.7471765405191 | | 1965 | 8.8779527559055 | | 1964 | 8.6429725363489 | | 1962 | 13.469985358712 | | 1961 | 9.5449172576832 | | 1960 | 10.517746662325 |
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