| Year | Value | | 2017 | 4.31 | | 2016 | 4.02 | | 2015 | 3.79 | | 2014 | 3.55 | | 2013 | 3.29 | | 2012 | 3.01 | | 2011 | 2.69 | | 2010 | 2.47 | | 2009 | 2.29 | | 2008 | 2.13 | | 2007 | 1.99 | | 2006 | 1.91 | | 2005 | 1.84 | | 2004 | 1.79 | | 2003 | 1.73 | | 2002 | 1.7 | | 2001 | 1.66 | | 2000 | 1.68 | | 1999 | 2.5199999809 | | 1998 | 2.5299999714 | | 1997 | 2.5499999523 | | 1996 | 2.5499999523 | | 1995 | 2.6099998951 | | 1994 | 2.6300001144 | | 1993 | 2.6300001144 | | 1992 | 2.6199998856 | | 1991 | 2.5999999046 | | 1990 | 2.5799999237 | | 1989 | 2.5599999427795 | | 1988 | 2.539999961853 | | 1987 | 2.4800000190735 | | 1986 | 2.4000000953674 | | 1985 | 2.3699998855591 | | 1984 | 2.329999923706 | | 1983 | 2.289999961853 | | 1982 | 2.2599999904633 | | 1981 | 2.25 | | 1980 | 2.2300000190735 | | 1979 | 2.2000000476837 | | 1978 | 2.1400001049042 | | 1977 | 2.0699999332428 | | 1976 | 2 | | 1975 | 1.9199999570847 | | 1974 | 1.8500000238419 | | 1973 | 1.7699999809265 | | 1972 | 1.710000038147 | | 1971 | 1.5800000429153 | | 1970 | 1.539999961853 | | 1969 | 1.4500000476837 | | 1968 | 1.4500000476837 | | 1967 | 1.5 | | 1966 | 1.5199999809265 | | 1965 | 1.4400000572205 |
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